[GCW]

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.

[rothers]

The Chip is nearly the same as the phenom ii 955 X4 http://www.ebuyer.com/product/190673. AMD just lock 2 cores as the X4 and X2 chips cost the same to make. Air cooled it runs at 29C idle and 51C under full load (Prime95 testing). Admittedly this isn't with a stock cooler but this is clocked at 3.5GHz Quad core. And I'm not saying this is perfect but it is quite cheap for what you can get out of the chip. http://www.overclock.net/amd-cpus/525772-official-phenom-ii-x2-500-club.html This thread shows what some people have managed to get it stable at. Even at 3.1GHz X2 it flys and for only £65 you really cant go wrong as this is cheaper than an equivalent intel.