6800 has shaders model 3 support, the x800 runs 10 fps faster (on average) on most games, both get more or less the same 3dmarks (if using the proper drivers, the latest ones make my 6800 unable to run nalu, the omegas pumps it 3000 3dmarks and make it able to run nalu on double monitor config...)

the cheapest one is your best bet, some drivers testing and tweaking after buying them is essential to make both of them perform.

my advice wait until autumn or winter and buy next gen or buy current gen at significantly reduced prices