I had the same problem with my old video card (Geforce 3 Titanium).
...and I steel have this problem with my new one (Geforce 6800 GT), but with not all the visualizations.
And I am a geek for video stuff, so I have installed all new drivers.
...err... I don't want to be nasty, but Visualizations are not very beautiful : this is the only thing that I don't like in MC..
See by yourself:
3D vis are better, but the viewport is not correctly resized in window mode.
Moreover, the cover art vis don't resize the picture to fit all the area
BUT I LOVE MEDIA CENTER ANYWAY!!!