The 6870 will be bottlenecked a little by your CPU, the 6850 less so.
There again, it depends on the game and resolution you intend to play at... Games like StarCraft and Skyrim put more load on the CPU, games like Crysis, Battlefield 3 stress the GPU more.
Both cards are good for resolutions of 1920x1200 with a lot of eye-candy turned on (AF/AA/etc.).
Going by your name, are you buying these cards in India, or elsewhere? Compare local prices (e.g. for India try theitdepot.com or ebay.in) and see what works for you.
Stick to good/reputed brands like XFX, Asus, HIS, etc.