People are going to think I am a VirtualBox evangelist pretty soon, but the fact of the matter is that I after paying for VMware Fusion I now use VirtualBox! I use it for WinXP and Ubuntu, and can't say enough good things about it. I'm running it on a MacBook Pro 4,1 with SL 10.6.2, 2gb RAM. Obviously more RAM = better performance for virtualization in most cases, but I'm well pleased with the results...