Today I tried to set up two monitors for my computer running 8800GTS GPU. I couldn't get it to work. I have the latest stable drivers installed and I've double checked the the control panel settings. The System recognizes both monitors but fails to display picture on both. It appears that one of the two DVI ports isn't working because neither of the screens work on it.
What ports does your graphics card have? If it doesn't have anything else than the two DVIs, I'm afraid you'll have to buy a replacement card (I suppose the warranty for your 8800 GTS has already expired?).