Since u already have a 7770, id say go for crossfire as what im doing with my 6770, i got another one since i didnt have the money to get a nice single card, except i bought both my gpus used for like $50 a piece lol
U could always try to craigslist or ebay the card and see how much u get from it, and get a 7870 or 7850, if u have the money for a new 7770 at 135, then getting $100 for the current one will be enough to get a 7870 or 7850. If u had the 2gb 7770 that only asus sells u prob could just get another since maybe ur lacking vram im not sure but there $160 a piece.
So ur decision on getting another card to crossfire, or sell ur current and get a good single gpu, Xfire 7770 are very similar to 7870 in bf3 performance, but Xfire in crysis 2 on ultra 1080p is about 20fps less than one 7870