As was mentioned XP 64bit would be the way to go (assuming you're running 64bit capable processors).
Keep in mind though that if you're running 32bit applications they still aren't going to get to use all 8GB of RAM. In the 32bit world there is (physically) only 4GB of memory space to map to.
why would you want to run XP on a system like that?
Maybe it's a high-end workstation?
32 bit Windoze has a 3GB per program maximum.
It's not so cut and dry, it's very application dependent. AFAIK there are only a handful of applications that are able to take advantage of the /3GB switch.