Well, in my experience, Win2K doesn't freeze. Programs freeze, etc. But the OS always kept right on running. I'm always able to hit Task Manager and kill whatever's annoying me. But I'm no Microsoft posterchild.

I love ATI cards, too (was totally giddy when I saw the ATI logo on the back of my new GameCube). But I do hear not so nice things about the drivers written for said cards. Beyond that, I would check to make sure everything is updated. DirectX, audio drivers, video drivers (I understand the card is new but it couldn't hurt to check), etc.

When you've exhausted anything you can think of as far as software goes... then it's time to start looking at hardware. So what would make you think it's the motherboard?