I am planning on building a PC with the following parts:
Case : CoolerMaster 690
PSU: OCZ Stealth XStream 700W
MoBo: Asus m4a79XTD Evo
CPU: Amd Phenom x4 955
Memory: 4gb ram
HD: 1tb Samsung Spinpoint
Optical: Blu ray reader + DVD RW
GPU: Sapphire HD 5850
The Pc is mainly for gaming, however I would like to change the grpahics card to a cheaper alternative. I want one that is still over 1gb, and has a HDMI output, but also for it to play the latest games almost as the 5850 would.
Well, if you want DX 11 then you either go with the slower 5770 or wait for the upcoming Radeon 5830 which may (or may not) come out early next month. As you know, the 5770 is basically half a 5870 so it's not fair to expect it to play games as well as a 5850. Your other option would be a Radeon 4890. While limited to DX10.1 it is a fair bit faster than the 5770, especially the overclocked versions, and thus currently fills the gap between the 5770 and the 5850.
If you don't mind a dual card solution, since you have a crossfire board after all, Then you may want to consider two 5750's, if you can find them on sale, or two Radeon 4850 1Gbs as they will provide you with comparable performance. It wouldn't be too much cheaper than a single 5850, but such a crossfire solution would be a little faster .
I'd go with the 5770 and potnetially overclock it.
What's your monitor's resolution? If it's not 1900x, you won't notice a difference between the 5850 and the 5770 at stock. If it is 1900x, there will be a difference at high settings, which should decrease with the overclocking.
Your quickest resources for this will be the Graphics Card Hierarchy Chart and perhaps the benchmarks on the Graphics Cards Charts 2009. The benchmarks aren't worth a ton without doing some research to understand why the results came out as they did, but they will give you at least some quick info.
Ultimately, if you want the performance of a 5850, get a 5850, get 2x4850 in CF. The 4890 and 5770 are going to be a step down in performance - perhaps not a very noticeable one, but a step down nonetheless.