I just upgraded recently after asking for advice here. I bought a used system. X4 965, 5770, 4gb ram ddr3. I upgraded that to 12gb ram, and it works superb using ISBoxer + 5 windows on 1 computer. I'm sure an I7 would be better, but I get no lag or hiccups anywhere, settings maxed on main window.

As mentioned in other threads, hard drive speed and plenty of memory will help you the most, then a quad core processor. Everything else is secondary. In my opinion, of course.