my monitor is 1680x 1050. What about the 5830? would it be a better deal?
The 5830 is not a good deal at it's current price point. If you get a 5830, you'll just wish you got a 5850 down the line, it simply destroys the 5830 in every benchmark, and a 5850 isn't much more expensive at all. And, if / when you do get the upgrade bug, a 5850 is simply unreal performance-wise in crossfire. For your monitor resolution, no, you don't "need" more than a 5770, but if you get a 5850 you are very effectively future-proofing yourself when you upgrade your monitor.