The 5800 series are better off in eyefinity and higher resolutions of gaming. 1920x1080 or higher resolution you would go with the 5830, but if you're playing games at a lower resolution then go with the 5770. Only problem with a 5770 would be that you can't play games such as Metro 2033 on max dx11 at a large resolution.
Also, what is your current cpu?
Edit: Wow, almost said what is your Thermaltake case D: