i would go with the 8800gts, which ever one fits ur budget. for ur needs, 8800gts 640mb. but if u comparing the 8600gts or gt and the 7900gtx, i woould go with the 7900gtx.
i think u should get the 8800gts 640mb, but some people may argue on that.
The 8800GTS beats the 7900GTX quite easily really. At that resolution you can just choose if you think the extra memory is worth the money since thats still not too high of a res.