it sounds like you have the memory maxed out for the OS you are using
if you can only upgrade on thing then it will have to be the video card.

It is my experience that the video card is doing a majority of the work so you should see a pretty hefty improvement with a faster video card.
the only possible drawback would be the 32 bit OS.

Try one with more that 256M , preferably 512M or 1G.