yes, there is probably already a thread on this, but perhaps there is new info now... Are third party visulizations available for itunes. My brother's PC has a great visualization that models the laws of physics. Come on Apple...
I really like G-Force for two reasons: 1) because it's different and 2) because it also comes with a separate app that will visualize the sound coming in through the line in jack.
If you don't mind paying $8 (I think) there is EyephedrineFX (sp?) which uses OpenGl and was really cool and smooth on my Mini. If you don't pay then it flashes some annoying message in the middle of the screen from time to time.