1. Download EVGA Precision here: http://www.evga.com/precision/

2. The main screen will look like this:


3. There will be no value for the Shader bar.  This is normal for 7 series and lower cards.

4. You can either drag the Core and Memory sliders over until the corresponding numbers are at least 5MHz higher than they were or you can click the numbers and manually type in values.

5. Click "Apply"

6. You can save the overclock as a profile by clicking the arrow below the Apply button so a little drop down appears with 10 grayed out numbers (1-0).
 Right click one of them and it will save the profile.

7. Click the little radio button above the Test button.  This will set Precision to automatically apply this overclock on boot.

Click the wrench icon to the right of the apply button.  In the main window:


Click the Start with Windows and Start minimized buttons and hit OK.

8. Enjoy your less problematic 7900GT.  Precision will start when Windows starts and will load the OC automatically.  If there are any other issues, feel free to come back and let us know.

Good luck!
 
EDIT: If you still have issues after this, I have a 7900GT that I can throw in my Windows 7 computer and try multiple monitors.  Just let me know.