The 520 will be plenty
Also, if your not overclocking, thats a pretty sweet processor, however if you think you might tinker to make it more future-proof, theres always the Q6600 to consider which could hit 3Gig epicly easily for about a fiver more and then your read for future with Quad core (also if you sell later Quad looks great to freshies buying in the market).
If your worried about heat, a heatpipe for your processor wouldn't be a bad place to start, you can get good ones for around £20, I personally prefer the Xigmatek 120mm Multi CPU s775 fan for £22
Don't forget an optical drive, £10/15 is plenty. Theres decent samsung litescribe satas on SCAN.
Also, you running a 64Bit OS on that? That'll be another £60... Plus a good thermal paste, £3/£4 something