Forgot to say: ProjectM is using OpenGL for Milkdrops, not DirectX. OpenGL version leans on your graphic card, not on the processor and memory, so it's very easy to play with it. DirectX version is known for a few bad bugs, so it's crashing it's hosts all the time (WinAmp, BASS plugins, etc). If you can make OpenGL plugin for PC users, it could be easily ported to Mac later. Hope this can help to someone...
If you want to try how smooth it plays, just install free Boxee software, play some mp3 and click on the arrow (upper right on the screen). If you want it fullscreen, click on Escape. It's just amazing on my Dell i3 with Intel's integrated graphic card. And if you want more Milkdrops, just find them on the web and put them in the Milkdrop folder ( c: program files/boxee/visualizations/milkdrop)...