Hey! Yeah, im was aiming for about the same and that started way before christmas, and my original budget was around £300 but no way that was going to happen! But saying that I had to buy Monitor etc. These are the specs I went with Mate;
Processor; AMD Phenom II X4 960T
GPU; XFX ATi Radeon HD 6770
Motherboard; MSI 870-C45 (nice board, overclocking switch.)
RAM; Corsair XMS3 4GB
HDD; Western Digital Caviar Green 1TB (nice 64mb cache.)
PSU; XFX 650w Semi Modular XXX Edition PSU (nice modular capability)
CPU Cooler; Arctic Cooling Freezer A30 (only released this week and looks sweet)
Case; Coolermaster 430 Elite (comes with nice blue led front fan)
There you go, I dont knowe the exact prices of these but they definately came under £500 and because of that you could probably upgrade some parts and it should run fine for casual gaming and media +web use (as I intend to do aswell).
The website I used is @Dabs.com, they are a nice, secure UK company who ship real fast. BEWARE! Do not buy from @WeAreElectricals.co.uk, as you may be fooled by their cheaper than average prices and snazzy website. I brought a processor from them before xmas, didnt come so I cancelled it. they said it was still in France (which they say to everyone else as I have read up on them in the Guardian and other places) I have still not recieved any form of payment and I have been told several times, by their shitey customer services, that I will recieve it the next working day!
You won't be able to overclock on this build, but sandy bridge is nippy anyway. The only problem i can see woudl be that the i3 2100 is only a dual core, it has hyper threading though so you'll get 4 threads.
I personally wouldn't buy from dabs as i think they're overpriced, 2 sites you could look at are scan.co.uk and ebuyer.com (and aria.co.uk obviously)
BF3 is quite a cpu intensive game and uses up to 4 cores. That's why i recommend the i5 which is a four core processor, you could swap it for an i3 to make it cheaper though. The reason is a bit more expensive is because this should last you quite a long time and this could max out any game on ultra settings