Hmmm, so WHY a 7870? Care to elaborate any?
EDIT: How reliable can I expect it to be? Also, can someone recommend a particular 7870 that I should get?
Why would you pick Radeon 7870? For the pure performance and the overclocking ability.
Why would you pick GTX 660? Because of the new technologies that it has FXAA, TXAA, Physx, boost, adaptive vsync. The low power consumption during gaming, which actually is lower than the Radeon 7850.
Radeon 7870 is probably around 5% ahead in performance more or less at stock speeds. It's hard to say when Nvidia just came out with new drivers 2 days ago.