To be honest, if the most you can spend is £150, I would go for AMD's side, FX 8320 or 8350. Because the new gen consoles have 8 core processors, console ports over to PC will be better optimized for more cores. Of course, right now an i5 3570k/4570k may give you a few extra frames, but if you're looking for future proofing I would go for the 8 core solutions from AMD. Just my two
cents pence
Of course, if you want an overall more responsive system i.e. faster app loading, better boot times and copying stuff from other drives I would go for the SSD (Jimbo's suggestion). It is the component I most regret not including in my current system right now
But back to your original post, if you get the i3 3220 (I believe that is the correct model), you may enable HyperThreading , which divides each core into 2 threads so 2x2=4 threads, so you pretty much get a quad core processor. When that is enabled, you really won't notice much
real world performance, only stats from benchmarks and you most likely won't notice 5 dropped frames.
Another point I would like to make is that, when you upgrade the CPU, the GPU will thereafter be the bottleneck of the system... just bear that in mind if you do not get a dramatic increase in games.
Hopes this steers you in the right direction! Choosing upgrades wisely can really be good for the short term and long term
