Good on you for getting a new gfx card. I'm holding out for the DX10 chipsets, for Vista.
Anyway, I had a problem like this, and the oddest thing helped fix it. All I had to do was underclock my gfx card a tiny bit. (This was when I first had it), its stock clock was 485, something like that, I underclocked it down to 479, and it was fine.
I've now overclocked it to 510/1450, and it is running fine, (since the problem I've updated my drivers), seems fairly random and I think it be a range of random things. What brand is your card btw, (sorry if you've already said!).
Charlie -