I would recommend the amd phenom ii 550 BE http://www.ebuyer.com/product/166518 as its good for overclocking and if your lucky and have a motherboard capable of it, then you can unlock the 2 dormant cores and you have yourself a quad core for around £65.
L3 cache is only 6MB though, so if you unlock your 2 dormant cores you may as well buy an old Q6600 which will clock to the same speed as it'll cost much the same. Although I note the power difference, so I'm not sure how hot your Phenom will run on air cool.