If you overclock here are the two options
IMHO:
1.)HD7850 - overclocked can hit around GTX580 level performance
http://hwbot.org/hardware/videocard/radeon_hd_7850/
Stock clockspeed is 860MHZ.
2.)HD7950 - can have over HD7970 and GTX670 level performance
http://hwbot.org/hardware/videocard/radeon_hd_7950/
Stock clockspeed is 800MHZ and the average overclock is 1165MHZ from 550 submissions. Even at around 1GHZ,you should be seeing HD7970 level performance
IMHO.
GTX670 is a better option if you don't overclock,but the HD7950 does seem to have much higher overclocking headroom on average. It is extremely underclocked at stock clockspeeds.