[this_is_gav]

512mb was usable on a laptop I was testing stuff out with and regardless of the supporting system 2gb should be more than enough for most.

4gb will work fine with Vista 32-bit, but as shaithis said, it'll only see 3.2gb to 3.8gb of it (depending largely on how much memory the video card uses). Any single process can only address 2gb max of RAM on 32-bit, which would likely be a larger limitation than the overall limit.

4gb on 32-bit would still be very nice, if only for SuperFetching (I got 8gb in my home PC for this very reason). It's not critical to have to have a 64-bit OS to take advantage of it, it's just ideal.