I had a similar problem on an older monitor. You can try two of the following things:
1) Right click on your desktop, then properties, then advance. See if you can adjust the brightness from there.
2) Download rivatuner, you can enhance the brightness from that little software also.
Asus P5W DH Deluxe
Core 2 Duo E6400@3.3ghz
2GB Mushkin Redline XP-8000
Hitachi 160GB Desktar
eVGA 7900GT Signature Series
Dell 2007FP WUXGA