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.
I would go with either the AMD FX-4170 Zambezi 4.2GHz (4.3GHz Turbo) Socket AM3+ 125W Quad-Core Desktop Processor FD4170FRGUBOX for $140. It has the 4 core/8 thread + it runs at 4.2 GHz. Or the AMD FX-8150 Zambezi 3.6GHz Socket AM3+ 125W Eight-Core Desktop Processor FD8150FRGUBOX for $220 with the 8 cores/16 threads if you're going to be doing a lot of rendering or multi-core oriented stuff. Like NerdRampage said, it really depends on the work/play load you plan to be putting on it.
One thing you really need to do is check the bios version you have on the board. Some cpu's require the F9 or F10 versions while some of them only require the F2 or F4. If you currently have a cpu for it make sure you upgrade to the version you need (or the newest which is F10) BEFORE you purchase a CPU!