Well they're pretty close. If you were to compare plain numbers, the X800 would win, but it lacks SM3.0 support (increases performance, 6600GT has it). Take that as you will, ATi has driver issues and the X800 series was regarded as extremely crappy, but from looking at it, sems the X800 would win.
How much can you spend? A 6800GS would out-do a X850Pro even.