Re: Gaming pc for $700
The 5770 and 4890 are so close in terms of speed that it's almost irrelevant. You're not getting a 4890 OR a 5770 for high-end performance anyway. The difference would be 2 or 3 FPS at most, and the 5770 has DX11 features that the 4890 doesn't have. Also, the 5770 will likely improve its speed with driver enhancements, and the 4890 isn't going to get any faster with future drivers.
I agree with VishKapish's suggestions. It saves a bit of money, and gives you DX 11 features at a minimal cost in performance (2-3FPS max)