I wouldnt try and run vista on 2 gigs
or 4
Vista 32 just seems to be a completely worthless OS. All of the pitfalls and resource consumption of Vista with none of the benefits.
You dont just need ram for fx (in fact many fx dont use much ram at all) you need ram for realtime edits, take histories, and especially sample based vst-is
Going to vista 64 would give you the ability to use a lot more ram, possibly enough to make it as fast as xp and have loads left over, provided a substantial cpu upgrade over the one you have in xp