Here is the verdict: On my desktop at home TVK and MusicBot-D (with visualization on or off) use up the same amount of CPU (0-2%).
My laptop runs on XP-SP1, my desktop on XP-SP2.
I think I found a bug: MusicBot will not unload itself from memory when you exit (you have to be connected to a room to make this happen), so you will have multiple instances of it running if you exit and re-execute the program a few times.
Cheers