at stock speeds the HD 7870 is on par with GTX 580. when overclocked to 1.25 ghz it matches HD 7950 boost (925 mhz) and GTX 670. the GTX 480 needs to be watercooled and and running at 950 mhz to 1 ghz to reach that kind of performance.
here is a golden msi hawk at 1325 mhz matching HD 7970 (1 Ghz) performance
also instead of a used HD 7870 you can get a new HD 7870 Tahiti LE and sell the game coupons to cut cost. you could easily get USD 50 from selling the 2 game coupons.
Tahiti LE cards
US NCIX has the Powercolor HD 7870 Tahiti LE for USD 249 (219 after mail in rebate) . check if us ncix has the game bundle offer for this card.
this card matches HD 7950(800 mhz) at stock speeds. runs at 975 mhz boost clocks. its 10 - 12% faster than HD 7870 at same clocks. easily overclocks to 1.15 - 1.2 ghz at stock voltage of 1.25v.Edited by raghu78 - 3/6/13 at 12:43pm