HD5970 ResolutionPproblem on Fullscreen

Hello,

I have a Powercolor HD5970 graphic card and I'm experiencing a problem that I could not identify. My desktop resolution is 1920x1080 and I have a Samsung SyncMaster SA350 monitor. When I try to play some games on fullscreen (1920x1080 res) the fonts and ui of the game looks like in a lesser resolution and the text are hard to read. When I switch to windowed mode with the same resolution (1920x1080) there is no problem but it is a huge performance issue to play on windowed mode. I would appreciate the help if you have any idaes.

Here are some information about my settings,
My monitor's hertz is 60 and same in game
I tried vertical sync on/off, nothing changes
I have the latest catalyst driver 12.10
My Crossfire settings is on Default
The current games I'm having the problems are WoW, Dota 2 and ArcheAge. I do not have other games installed.