Depends upon what kind of PC they run. Most Intels now run DDR2 and all AMDs run DDR, either PC3200 or faster.
Older Intels run DDR as well, in fact I have 2 gigs of PC3200 in mine.
Phenom II 940 Black Edition quad core @ 3.5Ghz
2x1 gig OCZ PC26400 Platinum, 2x1gig GSkill PC26400
Upcoming EVGA GTX470
Some other stuff,
Even more stuff,
If the automobile had followed the same development cycle as the computer, a Rolls-Royce would today cost $100, get a million miles per gallon, and explode once a year, killing everyone inside. --Robert X. Cringely, InfoWorld magazine