A single 560 Ti or 6950 would run fine with your power supply. I would recommend getting one of those over crossfiring a 5770. Not only would a 560 Ti or 6950 be faster, you won't have to deal with micro-stuttering that would be preset in a 2x 5770 setup.
Crysis Warhead, 5770 in CF: http://www.guru3d.com/article/radeon-hd-5770-review-test/18
Crysis Warhead, 6950 and 560 Ti: http://www.guru3d.com/article/radeon-hd-6950-1gb-vs-geforce-gtx-560-ti-review/11
Later on if you wanted to upgrade just get a new power supply and another 6950.