There was an article that I read at LostCircuits (which happens to be the site which was headed by the guy that went over to OCZ; I can't remember his name, but flapper know it) which did a rebuttal to an article at another site/magazine that stated that in certain cases, 2 gigs of ram hurt performance. The LostCircuits rebuttal did a good job of explaining the flaws in the first article and went on to show the advantages of more ram. If you are interested, you should be able to find it at that site without too much effort, just look in the Memory section. It is probably summed up best by saying that more ram won't hurt (unless you are trying for an O/C record), but it will only shine in those applications that require large amounts, such as video related apps. Although I am not a gamer at all, I hear that future games will require more ram.
Most people with AMD boards only have 3 slots, you on the other hand have 4 on your Gigabyte board, therefore, you could just go half way and get another 2 x 256 of PC3500 and at 1 1/2 gigs, you should have no problems playing current and future games.