A GeForce 6800GS or Sapphire Radeon GTO² will currently give you the most bang for the buck value at that price point... If you overclock the GS, it is about as fast as the 6800 Ultra, and by unlocking and overclocking the GTO², you have a X850XT (PE)... However, the latter doesn't support Shader Model 3.0, which is starting to become a bit more interesting right now...
There is still another solution, and that is to wait... Both ATI and nVidia are about to release their next-generation cards, and those may be interesting performance-wise, but they will almost certainly lead to lower prices on the current hardware... Imagine buying a 7800GT at the $ 200 price point...
So, in your case, I'd wait just a little bit, and see how the market evolves due to the introduction of the new graphics cards...