You can try 'scaling options' in Catalyst Control Center. Be sure you have the latest ATI Radeon driver for the 5870.
http://support.amd.com/us/gpudownload/Pages/index.aspx
Open CCC by right clicking on the desktop and opening the software app. Follow these steps for scaling options.
1. Click on “Graphics” this will give you a drop down menu
2. Click on “Desktop and Displays”
3. Click on Detect Displays
4. Select the TV or Monitor from the bottom list
5. A icon with a TV or Monitor will show up under the words “Please select a display”
6. Right Click on the Icon and select “Configure”
7. This will bring you to a new window
8. Click on the tab “Scaling Options”
9. Use the slider to adjust the screen accordingly to your preference.