Worth going from a 6770 to 6850?

I current have a 6770, and plays bf3 with my 6 core at 30fps on high preset and better with some medium settings tweaking, but my friend has a 6850 he'd like to trade and i offered him my psp or a paintball gun lol well i can get around $90 for either of those items and saw only a 10-15 fps increase with the 6850. Both 1gb cards. Wondering if it'd b a smart move or wait for someone to buy each item and get something like a 78507870 or 660gtx 2gb? Also was looking at some of the 7770s and i see the benchmarks for the 1gb cards but theres an asus 2gb version rlly similar to the 1gb ones and since wat i mainly play is bf3, would the extra vram help without a higher resolution, would it increase performance in a 7770? Playing at 1080p