32Bit O/S can only handle 4gb total, and thats including Video card ram and other stuff. So if you have 4gbs of ram and install 32bit, and have a 512mb video card. Your most likely going to only show 3.25gbs of ram (thats how much I have under 32bit W7).
So if you have 3gbs of ram under 32bit, you should get all of your ram, unless maybe if you have some insane video card setup with a couple cards totaling 1+gb of video ram.
But Yeah if you ever get 4Gbs of ram, I'd suggest Vista 64bit (since I'm running that now). I've only have one driver problem and that was with my TV tuner, and Its the companies fault anyway (damn Pinnacle, worst company ever!).