A small correction to the original post:
Alienware used to base all of their computers on the Intel Pentium/Celeron chip. However, they've recently generated a large number of sales based on the AMD Athlon/Duron chip. Their flag-ship computer (called the Area 51) is based on the fastest available CPU. This has flip-flopped between the AMD Athlon Thunderbird CPU and the Intel Pentium III CPU. With the debut of the Intel Pentium 4 chip, the Area 51 configuration is currently based on that chip.
An examination of their other gaming configurations (Grey, Hivemind, Aurora) show that they are all powered by the Athlon CPUs (Duron, Thunderbird).