Adobe seems to be advocating for 64-bit OS installs, and watching to see if there's any push back. Michael Coleman is asking for comments on what’s holding people back from 64-bit in his PVC post, in addition to explaining the performance benefits already built into CS4 Production Premium.
Windows users seem to be the ones lagging behind since the big cutoff for the Mac was Intel-only CS4 (OS X 10.5 is about a year and a half old). After Effects plug-ins could be an issue for some people. While rumor is that 64-bit XP works nicely, only Vista is supported by Adobe and by 3rd party hardware solutions like AJA. Based on a small sample of past releases, CS5 could be another year away so there's no rush. If you're looking in retails stores, the 64-bit Windows boxes are the ones with 4+ GB of RAM (except the next-gen Intel Core i7) -- and there are some not expensive portables with DDR3 RAM or 1GB nVidia graphics.
Update: Jan Ozer did some tests comparing 32- and 64-bit OS performance for EventDV a few months ago:
"After correcting for the 18% difference in processor speed, the 64-bit system was 67% faster on my standard DV test file, up to 63% faster on HDV-related tests, up to 50% faster on AVCHD tests, and up to 227% faster on tests using footage from the RED camera. The only format that didn’t seem to improve on the 64-bit system was DVCPRO HD, which showed only a 13% speed boost."