Any name-brand manufactured computer isn't going to have much room to upgrade. You have 2 things you can really do. First, upgrade the RAM to 2G, and upgrade the 6600GT to a 7600GT (best AGP card you can get, since they stopped making them). Other than that, you aren't going to get much better.
If your monitor is the Sceptre Naga widescreen, you are going to have to replace the entire system to get framerates up when using 1680x1050 resolution. An Athlon XP isn't going to last you much longer, since the video card is pulling all the weight for you.
If you have the money, I would suggest building a system, and not buy another name-brand system. This way you can upgrade the motherboard, and everything else, and not be stuck with a small form factor motherboard. Not to mention, it's cheaper in the long-run, and you aren't buying el-cheapo parts (there is a reason those systems are so cheap).