The 5770 performs similarly to a 4870, a GTX275 performs similarly to a 4890. The 5770 may get improved performance through driver improvements but in DX10 games the GTX275 will likely remain better.
practically GTX275 will be better. in recent benchmark it shows that 5770 is in HD4870 / GTX260 range performance and GTX275 is clearly better than this two. but the HD5770 supports new dx11 and the GTX 275 doesn't have that. if you want better performance either CF the HD5770 or get single HD5850