7770 is overpriced, beside you will need more than 1GB VRAM to play bf3 on ultra at 1080p. Might want to look into Nvidia 600 series GPU. They perform much better than ATI 7000 series.
Two 7770 will probably give you performance of a GTX580. But you probably wont be able to get 60+ FPS on ultra. Just play it on high, you don't notice that much of a graphic improvement from high and ultra.