Sorry if this is a stupid question, but I am intending to move from a PC to either a Mac Pro or an Imac 24"., and whilst researching the options, I keep seeing different processor names for the Mac Pro - Quad Xeon, Harpertown (and one other - I can't remember the name).

I understand that there is a water-cooled processor somewhere in the range which I have been warned not to get, but apart from that, is there any real difference, and is any given processor better than the others?