For just a little more, I highly recommend the FX-6300. It offers superior performance and is a lot better for overclocking than the Phenom, which has a relatively low OC ceiling. With the right cooling equipment, it's pretty easy to get to 5 GHZ on the FX Series CPUs. So The 4300 is better than the Phenom, but the 6300 is better than the 4300.
6300 + a £15 Arctic Freezer 7 is probably a better option than the 4300 + CM Hyper 212. They are both silent, and you will be able to OC further with the 212, but the extra clock speeds won't make up for the performance gap. It would be close anyway, you won't get as far on a 212 as you think.