The X850XT was the competition for the 6800 Ultra which it beat in most games. Performance should be close to identical to what you were getting with your overclocked 6800GT. But you lose Shader Model 3.0 support.
$287 for an X850XT??? That is way, way too much. Hell, you could buy a PCIe 7600GT and a S754 PCIe motherboard (like the DFI Infinity) and have better performance!
I think a used card would be best for you. Having used a 6800GT, stepping down to an AGP 7600GS/X1600 wouldn't be a good idea...and moving up to a 7800GS has way too high a price premium. Try and look for another 6800GS/GT or an X850XT, I guess. I'm not good with optimal pricing for used-cards but it has got to be around $120, I think.