I recently did the same upgrade, I bought the Asus radeon 7850. With the fantastic GPU Tweak software they include I oc'd the gpu from 870mhz stock to 1101mhz! I ran the heaven benchmark and it is within 1 fps of a radeon 7950!! The nice thing about the gpu tweak program is that it can link the voltage to the core clock, so you dont have to guess the voltage. It made overclocking a no brainer. The dual fans are very quiet and cool. With the user defined fan profiles in the Heaven benchmark the temp topped out @ 82c with the fans @ 50%.
I did consider the 7870 but its $100 or more than the 7850 and I did not feel that it was $100 better than the 7850.
I must say this was a great purchase. Here is a
review of the card for your consideration.