What motherboard do you have, will it support either SLI or CF?
If it only supports one video card your best bet is probably either GTX 260 (unless you just have cash to burn, then go for the GTX 280) or 4870 + aftermarket cooler and OC.
If you have the option to use a multiGPU setup, you can get some real power for fairly low cost these days.
SLI: Add a second 8800GTX or sell that card and buy 2x9800GTX ($400, beats GTX 280 nearly across the board).
CF: Buy 2x4850 ($350) or 2x4870 ($600, best performance you can get today at this price point).