Up to 8? I'm sure you can do more than that. I've seen people with 12 or more.
When I was at my peak, I had a quad core, dual core, and an ATI graphics card folding 24/7. Sadly all thats gone. And right now, I'm having trouble with this SMP, always ending early with my E2140.
And motox's advice on cranking out as many WUs, is the way to do it.