The 5770 costs a little over half the price of a 5850, and performs a little over half the performance of a 5850, so they're pretty well matched to their price points. However, it will be harder to upgrade to crossfire 5770s than if you bought a 5850 in the first place. If you have the money, 5850 ftw.
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.
This is the only HD5830 that is priced right based on performance http://www.newegg.com/Product/Product.aspx?Item=N82E168... $199 after MIR. Going from a HD4850 to a HD5770 is not a very noticeable upgrade so a HD5850 is a better choice unless you want to go for the HD5830 which should be a noticeable upgrade.