There isn't a single chance in hell a AMD + 5850 won't blow away an intel with a 5770, I mean, not a chance in hell.
The key thing you want, is gpu, fit ANYTHING in around the best gpu you can afford.
Personally, I'd grab a 555Be, you have an amazingly high chance of unlocking the core to a quad anyway, save some money and have money left over for a quality cooler.
Even if you stick with a quad I'd certainly buy a nice big but cheap cooler.
Make sure you get a mobo that can unlock cpu cores and you're golden, I'd also consider a last gen chipset mobo, the 7xx boards as they are cheaper and, sata 3 won't the slightest bit of difference unless using a top end ssd which he won't be.
A £80 cpu, a £55-70 760/780 mobo or something, and a £30 cooler and you'll probably end up with a perfectly easy perfectly stable 3.8Ghz quad core for quite a bit less.
I'd also probably swap to a 500W OCZ modular psu, because you're going for a small case and its classed as silent, aswell as getting rid of cables you don't need making it a bit neater. Theres a 500W ocz non modular one for £46 aswell which is more than you need.
Any mobo you buy will have a bunch of sata cables so no need to buy one.
So save money on mobo, chip, psu, buy a cooler, overclock it(overclocked + decent tower cooler will run cooler and faster than with stock cooler as the stock cooler is pants and quite loud). I'd also consider spending a bit extra and grabbing maybe one of the antec 300's, something a little better is likely to come with much higher quality fans, heavier panels that vibrate less and supress more sound and simply a higher quality finish/contruction and its only an extra £10.