Earlier today some preliminary benchmarks of GeForce 7900GT and GeForce 7900GTX were leaked onto the internet pending the March 9, 2006 launch.
Athlon 64 FX-55
1GB DDR400 (2x512MB)
All tests were performed with a 1280x1024 resolution. The cards tested claimed 1.2ns DDR3 for both cards. ForceWare 84.12 was used for testing.
GeForce 7900GTX 512MB
650mhz core clock, 1600MHz memory clock
3dmark 03: 17800
3dmark 05: 7600
3dmark 06: 5200
GeForce 7900GT 256MB
450mhz core clock, 1300MHz memory clock
3dmark 03: 13800
3dmark 05: 7000
3dmark 06: 4550
Looks to me like the 7900GT is the best bet, performing just under the 7900GTX, it cost a LOT less ($350 for 7900GT and $550 for GTX)