If you are looking to do more gaming, you'd be better off going with 4GB of ram and a newer graphics if that is within your budget. The difference between prices in the 4GB and 8GB ram modules would allow you to buy a gtx 460 instead of that 9800gt.
Agree with dassox. For a gaming machine, video card is much more important to run high details at 1920x1080 than more RAM. 4 GB of 1600 MHz CL7 should run about $100, like this G.Skill ECO Series kit. That's $150 saved over the Dominators, and better spec RAM than the XMS3.
For $170, you can get a 768 MB GTX 460, which will run ok at 1920x1080, if not amazing. If you haven't bought the 9800 GT yet, you're looking at about $250 (?) available to spend, which could get you a 1 GB GTX 460, a 5830, or it would be very close to a 5850.
Alternatively, if your motherboard supports SLI, you could consider getting another 9800 for the time being for around $100, and upgrade your video card(s) down the line.