It's got to be your card. I'm running the very same card and I had some probs...only you said your SP works fine. I haven't attempted to play this game on MP yet. But try visiting NVidia's site, and download their latest video card drivers. Download that Detonator XP thing. Here's a link to their site:
http://www.nvidia.com) Hit download Drivers at the top of their page, select your OS, and then pick the Detonator XP on the page it loads up. Also completely uninstall your previous video card drivers first, I learned the hard way that I had to do this in order for this to work. Reinstalling DirectX 8 or 8.1 after this probably isn't a bad idea as well. Hopefully that'll help.