Ivy Bridge is Intel's latest/greatest line of CPUs, about 10% faster than Sandy Bridge on average, although in some specific tasks it's 15% faster. Not a big deal and if you're on a budget, probably not worth the higher price.
If you're just doing everyday stuff like schoolwork, Facebook, music, watching movies, and occasional games, get a Core i3 2120. That's more than enough for 90% of stuff, better than all of AMD's processors.
If you're a heavy-duty gamer or doing occasional movie editing then go with a Core i5 3450 (currently out of stock) or 2400 and save £20 compared to the cost of a 3550 or 2500K.
Your performance in games depends mostly upon your graphics card, not your CPU anyway. And if you aren't running a high-end graphics card you don't need a 650 watt power supply. A good 500-550 watt power supply is enough for any midrange gaming card.
http://www.scan.co.uk/products/550w-psu-xfx-pro-series-core-edition-p1-550s-ukb9-85-eff-80-plus-bronze-sli-crossfire-eps-12v-fan-at
Antec, XFX, Corsair and OCZ are top-notch brands in the power supply department. The PSU linked above is a much better quality than a 650W Coolermaster (and costs less, leaving you with more money to put into your graphics card).
For a graphics card, I recommend the GeForce GTX 550 Ti. It can play all mainstream games on high settings without any issues, and it's affordable:
http://www.scan.co.uk/products/1gb-asus-gtx-550-ti-4104mhz-gddr5-gpu-900mhz-shader-1800mhz-192-cores-dual-link-dvi-i-d-sub-hdmi
Good luck!