Whoah, watch it there. Enabling DMA without doing some intense
testing afterwards can result in severe
data corruption, or a complete lock up of the
disk driver.
Some hard drives don't know how to do DMA well when connected to some controllers. Therefore, it is a good idea to always test, using heavy reading and writing on the disk when it doesn't have data you care about on it yet, before assuming your turning on DMA was successful. I had an old Quantum Fireball drive which would appear to work fine when DMA was turned on (and even perform faster) but would eventually lock up completely, requiring a hard reboot and a rather messy fsck.
When tweaking for performance, test, test, and test until you can't test anymore. Then, make sure you have good backups. }:).