in windows xp having more than 1.5gb of ram does nothing. Seriously i went from 1GB to 4GB ram in xp like a year back and i was all into it, ran benchmarks, recorded difference in gaming and encoding fps. the only thing that was different was my encoding and it only went up 4fps.
On the other hand windows vista x64 has a much better coding method and address the memory alot better and people see huge increases in ram performance (2Gb to 4GB)..