Here they are in order of performance:
GTX 295
HD 4870 X2 $400
GTX 285/GTX 280 $275-$330 (when the performance difference is under 5%, I group them together)
GTX 275/HD 4890 $235
GTX 260 Core 216/HD 4870 1GB $175
GTX 260/HD 4870 512MB $165
HD 4850/9800GTX+/HD 4770 $100-$130
HD 4830/9800GT $85-$95
I could go lower but do I really need to since your budget is $800-$1000?