The 6870 is more or less the same performance as a 5870 give or take a few % depending on the game.
Just go with a 6870 and save your self the money matey.
i7 970 - NH-D14 | Gigabyte x58a-oc | Corsair AX850 | CM Cosmos S
6Gb G.Skill Trident - 1600Mhz c7 | SLI GTX 580
Xspc Rasa | Phobya G-Changer 240 | Phobya dc12-400 | 7x Orange Xigmatek XLF 120mm | 2x EK 580 blocks
360 G-Changer | 200mmish Tube res | Orange coolant | Alphacool Copper Barbs, Spring clamps + CPU Screw kit