I'm thinking you should try to shoot for the 660 or 7870 in this case because both are weak cards for gaming and imo can hold you system backk
7850 weak for gaming!!!! maybe above 1080p with all the bells on whistles on. but for most gamers it would be very sufficient, and is a great overclocker too.
660 is also a reasonable card. but out of the two you mentioned the 7850 is a decent performing card that will play the likes of BF3 on high/ultra, along with your system you would get very nice frame rates.