If you're going to sell your XBOX, might as well get a goo card.
if you're around the 170 range, get a Radeon 6870.
If you're around the 200 range. Get a GTX 560 Ti.
Either can play Crysis 2 max settings no lag. But the GTX 560 is MUCH faster. And can play the BF3 beta on ultra no lag.
I don't know how you got it in your head that the 560Ti is way way better than the HD6870. They are near equal on most benchmarks, with the HD6870 coming out at 5 to 10% lower performance depending on the game/benchmark, while beating the 560TI on games such as F1 2010, and still being $30 to $40 cheaper.
Anyway, Dropshock, great build.
I'd suggest at least an HD6870 but your HD5770 will hold up fine until then. Remember, the HD5770 is also the HD6770, they just rebranded it. It does well for a cheap card.
Your 600W PSU will be heaps.
Just keep on savin' for that GPU :D