I was in the same boat until about a month ago. I've now got 4GB running in Vista home premium 32bit. It hasn't really made any noticeable difference in perfromance, but then again I haven't run any benchmark comparisons yet. I did get a noticable better frame rate in supreme commander and crysis.
Basically, I wouldn't stress too much about only having 2gb. DDR3 prices will go down over time, especially with nehalem + X58 (which ive heard is going to use tri-channel DDR3) release coming before the end of the year. so if I were you, just wait until next year before stepping up to 4gb or 6gb. remember also that if you are running a 32bit OS, you will only be able to use about 3gig, not the full 4gig. in order to use more than 3 gig u need a 64bit OS.