Bean0, you can have up to 4GB of RAM on a 32 bit operating system. Graphics card memory size is, AFAIK, unlimited and shouldn't stack with RAM (but the Graphics Card may still be limited to 4GB maximum size, however I don't think there's a graphics card out yet powerful enough to test that theory. lol)
Of course under XP, if you still prefer to run it (like me) then you will only get 3.5GB RAM due to the unique way in which Microsoft works. Good times.
Anyway, 64 bit is almost universally supported now so there's no reason to not get 64 bit.