The ATI 6790 is around 130$ but a much better value is the 6850 from ATI which is around 150$ (not including Tax or Shipping)
I have a 6790 right now and I get fantastic performance in most games at 1680x1050 on high settings. Sometimes I have to turn down the shadows and no AA but everything is 40 FPS+ minimum.
The 6790 stock clocks are 840/1050 I have mine Oc'd to 950/1205 and it makes a 5-10fps difference in most games. Although on the stock cooling it was at 60-70*C which is fine but hot IMO so i bought a 3rd party cooling solution that brings it down to 45*C under load on average.
I think you'll find that you get hung up on RAM shortages and the CPU abilities way before the GPU reaches maximum potential.
PS: I know nothing about Nvidia solutions so some one would have to jump in to give you that side of things.