This is the card to get, the 6800GS. I will buy that in a second if I need it. Priced very well for the performance. With the debut of the Geforce 6800 GS, expect the Geforce 6800 and 6800 GT to slowly disappear. When you look at the specifications, the 6800 GS very closely resembles a GeForce 6800. The differences are higher clock speeds that put the fillrate on the GS right under a GeForce 6800 GT (5100 MPixels/second for the 6800 GS and 5600 MPixels/second for the 6800 GT). The memory bandwidth offered is also right on target with what a GeForce 6800 GT offers at 32GB/sec. This means that there is a lot of fillrate and memory bandwidth being offered here for the price of around $250ish. Basically, we have what amounts to a GPU that is just shy of being a 6800 GT if you are just looking at specifications.....Anyways, I had gloat over the card first.
You will need PCI-e support though and I'm not sure if your system has that capabitlity knowing that you're running on an ancient Athlon XP nforce or nforce 2 chipset.