Just upgraded an XP computer to windows 7 and was wondering what a good amount of RAM for a gamer is. I'm not over clocker and don't worry about shaving off every fraction of a second, but if more RAM would make a difference I'd be willing to increase. I've got three gigs of DDR2 PC2 6400. It is in three slots out of four available on a Giga-Byte GA-EP43-UD3L.
If three is a bit too low the easiest thing would be to go to four with another gig, but if I need to shoot for six I'm not opposed to pulling one out and adding 2x2 (or adjusting the entire set up if that would be best).
With only three slots full, AFAIK you are not getting dual channel operation, significantly reducing memory performance. Memory sticks should be used in (matched) pairs on your mobo to avoid this. The specs for all memory sticks should be the same to avoid all memory operating at the speed of the slowest.
4GB is a good place to be, though many prefer 6GB to ensure 4GB is available for "gaming".
If your existing 3 sticks have the same specs, then adding a matching 4th stick will get you to 4GB and restore dual channel operation. If two are the same spec, then adding 2 matching 1GB sticks would also work (discarding the outlier).
Whichever move you need to make, compare it to the cost of a 2x2GB kit which absolutely will get you to 4GB and dual channel operation. Then make your decision.