Don't do it. Really.
Modems: Everyone has some idea of what these little boxes or expansion cards do: let your computer talk to other computers over telephone lines. On some, there are little lights that let technicians know what's going
on (but for most of us they just do a flashing dance while we are downloading web pages). Every modem has a phone jack on the back, where we plug in the silver cord that goes into the wall -- and to the Internet.
There are lots of kinds, but in general we just talk about the newest ones: 56K or 33.6. The fancy box that Earthlink sent to use the blazing-fast DSL line is also a modem. Here's something I bet you didn't know: the CB
radio in your car is also a modem. And so is the mobile phone in your pocket. And the FAX machine you just used to send that contract across town.
"Modem" is a shortened form of the words "modulate" and "demodulate" stuck together. That is to say, it is a device which modulates and demodulates a signal. What the heck does that mean? Well, it combines useful information
(e.g. your voice or computer data) with a predetermined signal and transmits it (modulating) or it extracts the predetermined signal it receives and spits out your useful data (de-modulation).
The parameters of the predetermined signal depend on the medium through which you are communicating. If you are sending radio waves over the air, there are an unlimited set of frequencies that can be used -- but the ITU and FCC regulate these frequencies into groupings to avoid international communication chaos. By stating that X megahertz will be used for television broadcasts and Y kilohertz will be used for air traffic control and Z hertz will be used for remote-controlled toys, these organizations make sure that devices that are supposed to do a particular task are able to do that particular task.
Over telephone lines, however, there is a slightly different circumstance. You could use any signal you want to exchange modulated data over a single wire (based on certain assumptions about the length of the wire, but capacitance is a whole different ballgame). However, telephone lines aren't any old piece of wire. In order to allow Alltel (your local phone company) to communicate with AT&T (your long distance phone company) to communicate with Sprint (your friend's cellular phone company), the ITU and FCC set up a bunch of standards for how the telephone company's equipment must format their signals. Even though the piece of wire between you and Alltel is a regular piece of wire (in actuality it isn't, but we can assume it is), not every signal you transmit over that wire will be guaranteed to be transmitted to AT&T and then to Sprint. In fact, there is a very narrow set of signals that will be transmitted, even if Alltel were connecting your call to your next door neighbor.
The local telephone company will take any signal from you up to 4000 Hz and send it to another part of their network or someone else's. At 4000 Hz, not many computer bits can be transmitted but human hearing works great. Voices are almost universally under 5000 Hz and even though many people can hear up to 20,000 Hz, the telephone standards were set with voices in mind. As a side note, this is why the music on answering machines and on-hold systems sounds so crappy: music uses our entire hearing range and when limited to 4000 Hz, a lot of the sounds are lost or altered.)
Using (now) archaic forms of modulation, the fastest you can transmit computer bits (or certain other things like telegraph clicks) is limited by the rate at which you can modulate your signal. In the case of an 4000 Hz telephone line, you can send one bit per cycle, or 4000 bits per second. The better part of a generation of geeks knew this pretty well and were certain that computer modems would never transmit as fast as 9600 bps, which turned out to be a popular standard for data transmission and is still the speed used by many fax machines. The way to get more data into this signal involves encoding, in addition to modulation. Thanks again to the ITU and FCC, we are limited to 8 bits (or in some cases, 7 bits) of "resolution" for every one of those cycles -- although the telco equipment reads the signal (your voice or modem data) at twice the 4000 Hz, taking 8000 samples per second. This is where the 56K comes from that is printed on your modem box or listed on the Dell website when you ordered your computer (if you read the fine print, it says somewhere that the modem is capable of 64 Kbps, but the FCC regulations limit it to 53K; this is related to the way local telephone company talks to the long distance company -- don't worry about it).
How does this relate to overclocking your modem? Let's assume for a moment that you live in an ideal world. (NB: You don't.) If your modem is transmitting data at the ITU- or FCC-mandated 4000 Hz, the telephone company will handle it wonderfully, doing its own signal conversion and passing the modem signal to your ISP's computer. Now, if you speed up the transmission rate a bit without telling the phone company, every few cycles you will send two cycles when Alltel is expecting one. You won't really know the signal was lost, when it was. There's a famous mathematician named Nyquist who has a lot to say about this. If you're having problems picturing this, think of walking down a flight of stairs. Instead of moving your foot 1 step width at a time, move it 1.2 step widths at a time. When you move a foot for the fifth time, your next foot will land a step down from where you are expecting. We've all done this inadvertently, and usually it results in a bit of back pain -- or worse, falling down the stairs. That step you missed is similar to the data the telephone company missed. (In this example, you are the telephone company, running at a lower frequency than the stairs.) This phenomenon is also responsible for why wagon wheels spin backward on TV and why fans spin in slow-motion when you turn on the strobe light at your Halloween party (And, interestingly, why diagonal lines on your computer monitor are jagged).
So in that case, "overclocking" your modem is bad if only for the reason that it causes more problems than benefits.
Some people, however propose other methods of "overclocking" a modem by recommending changes to software settings (such as configuring your Windows registry to only allow "115200K" connections. These methods are misleading at best as rates such as 115200K are not representative of the communication speed between your modem and your ISP -- they represent the communication speed between your modem and your computer. Modems and computers communicate at a fixed speed, to allow for more efficient transfer of information (since the pathway between the modem and the computer is consistent, it is almost an "ideal world"). The communication between your modem and your ISP, however, is not so lucky as to be anywhere near ideal conditions. Your modem manufacturer knows this. Your telephone company knows this. Your ISP knows this. The ITU and the FCC know this. Because of the less-than-ideal conditions of telephone communication, modern modems (type that five times fast) are built with the ability to speed up and slow down based on how well the signals are being transferred (or "renegotiate" to use the terminology your modem manufacturer advertises). All the while that this is going on, the speed between your computer and modem remain the same -- the computer never knows about these renegotiations.
In some modems, there is a setting (accessible via "Hayes compatible" AT commands) which specifies the speeds the modem will comunicate with other modems at. This type of configuration is not often accessible to the computer user, and is completely independent of any software settings. The format of the AT command is usually manufacturer-specific and is intended only for diagnostic purposes. This would seem like a great way to speed up your connection, but it comes at a price. If your modem is continually trying to negotiate a faster signal rate when the telephone line conditions do not support it, you can lose data. Today's modems, in addition to featuring auto-renegotiation features, also have the ability to detect when data is lost or corrupted and request that it be retransmitted. If your modem is operating on the edge of its performance threshold (in other words, "faster than it would have picked on its own"), this data loss can result in effective speeds that are slower than would have been obtained if the modem were left to its own devices.
In other words, "overclocking your modem" is something you cannot easily do without modifying the electronics of your modem. Anyone who tells you otherwise should never again be trusted for computer advice.