With even 4 GB, your best bet would be to run 2 as physical and 2 as a ramdisk. Why 2 and 2? Windows won't disable the swapfile no matter what you do, and you need more than 1GB due to formatting losses. This leaves about 800megs for temp files as well for whatever you are running. this way the machine will run 100% in memory and never tough the HD unless it's saving or loading something.
8 is plainly overkill unless it's going to be stuck in something like an I-Ram.
Edit - Vista 64 is nice but I'd just wait for Windows 7 in a year and a half. As it is, nothing short of stuff Pixar and Northrop are running uses more than 3-4GB anyways.
Oh - yes, you CAN disable the swapfile in Vista. It gives it a serious headache, though, as it assumes it's there - so this "trick" is a good way to kill two problems with one stone as it were.