I too have the HD5770 and I got the problem because my monitor's native resolution is 1440x900 (16:10).
The only solution that lets you keep lightning settings at high is using a 16:9 resolution instead. My monitor supports 1360x768 but it looked bad because it was being stretched so I decided to create a custom resolution that matched the pixel width of my monitor. In my case the resolution is 1440x810 but you'll have to do the math for your monitor. I get letter-boxing but it is minimal.
Instructions for creating a custom resolution with ATI can be found here:
http://www.ehow.com/how_7649449_add-custom-resolution-ati.html If you update Catalyst the registry setting will be deleted so you'll have to create it again every time you update.
Nvidia has a setting to do this in its control panel but I never got it working when I had my Geforce 8800 GTS. I could create the resolution but it had pilar-boxing as if it were 4:3 and desktop panning because the black bars wouldn't let the whole desktop show.