I've just swapped my ATI card for an nVidia (GT545) and ran into this problem.
Good old VGA cable into a CRT (keeping it old school).
This could get somewhat irritating
DVI - HD15 is the normal cause of this.
Edit your client_settings file and adjust the native height and width and refresh rate to your monitors maximum, the issue will be that if the monitor is not being reported correctly in the dxdiag then these settings will be a very high number.