I would like to upgrade my 7800GT. Low cost/Nvidia as preferences. Recommendations? (Model rather than series)?

I'd look for a GeForce 9600 GT or 9600 GSO. They can be had for around $100 usually less. The are basically rebaged GeForce 8800 GS's. If you can spend a little more. the GeForce 8800 GT is a good choice. The GeForce 8600 GT w/ DDR3 is pretty solid. but the 8600 series boards with DDR2 are worthless. As far as resellers go, I've had good luck with Evga and BFG. Keep in mind your CPU will become a bottleneck with any of these cards. Good luck.