Dunno, a 32 bit os will support 4gig of RAM, though it might look like you can only see 3.25-3.5 gig as it takes into account any other memory (GFX cards etc) the maximum alowed is due to the addressing. Its still there and used.

I couldent see why theres a difference between vista and XP, though in XP you can throw the 3 gig switch, not too sure if you can do that with vista.

I use xp 32 bit with the 3 gig switch with 4 gig of ram, no probs.

I am thinking of moving up to 64 bit in the next year or so.