I've got 3 high end computers, and I'm tempted to pick up 2 more, and try my hand at 5boxing, specifically, I want to be able to do almost all of the heroic 5 man instances eventually.

I'd like to keep the class types simple, so I'm leaning towards 1+4. Can you really do 1 priest + 5 warlock, and still do the 5 mans, or will you need to have some kind of tank?