I'd go for the 6950.
At your res you can probably just run a single 5770 and get playable framerates. But the 6950 may unlock into a 6970 (2gb version I believe has the chance of unlocking) and it scales with crossfire extremely well, whereas the 5000 class of GPUs don't scale as well.
5770 crossfire may be overkill at your monitor resolution, even the single 6950 is overkill. But if you ever upgrade your monitor to a higher res or add other monitors then you'd be getting the best bang out of your buck.
In the future if you require more power you can just pop in another 6950 (or 6970) and get fantastic performance.
Go with the 6950, 2GB or even 1GB version. Great video card and you'd be future proofing more than opting to go the 5770x2 route