Well, to be honest this depends heavily on what you want to do with the computer. With that kind of mobo I'm assuming you want to do something more on the heavy-duty side of things, such as graphics-heavy gaming. With that in mind I'd recommend the AMD FX-4100. It's about $100 (roughly 60-70 pounds), and it'll give you quad-core and 3.8ghz with turbo-boost.