Most games use 2 cores. The I5-3570K is great for most multi-tasking. Unless your are consistently going to run a lot of CPU-heavy tasks while gaming, the 3570K is the best option.
Here's a quick snippet from an Amazon review of the 3570k:
"I got this, and was playing Skyrim full screen with windows task manager, permformance tab, showing on my second screen. None of the 4 cores seemed stressed out and the game was silky smooth. I then played skyrim with a hd movie playing on my second screen and skyrim didn't skip a beat"
And another one:
"I've recently bought this CPU, upgraded from an e4500 @ 3GHz, and threw everything I could at it, installing programs + games (steam), running downloads, running furmark, playing a 1080p video and a full virus scan...*all at once* and it never even blinked!"
I own one myself and it's great!
Here's a link:http://www.amazon.co.uk/Intel-Generation-i5-3570K-3-40GHz-Technology/dp/B007RUZKK6/ref=sr_1_1?ie=UTF8&qid=1354715439&sr=8-1