£600 ish Gaming PC

Firstly, don't even bother including price's in $$ if your buying within the UK. Prices are very different and the exchange rate will never balance out.

Websites to buy parts from:
www.ebuyer.co.uk
www.amazon.co.uk
www.scan.co.uk

Do you require an operating system in your build? Will you need to buy Windows 7?

Ok so now the build:

CPU: i5-2500k - yes without a doubt get this into your build

RAM: 1600MHz RAM, 8GB (2x4GB sticks), Corsair Vengeance tends to be the best priced in the UK ~£40

PSU: 550w XFX Core Edition (Bronze rated)

GPU: GTX 560Ti/6950 (maybe next tier up will see how the budget goes)

Mobo: Z77 chipset is going to be your best option, ASUS would be my brand recommendation

HDD: This really depends on how much storage space you need, look for 7200rpm drives though, 500GB currently priced around £50-55 and 1TB I've seen a few deals for around £65 ish

DVD-RW: Something basic, ~£15 mark

Case: Don't waste too much of the budget here, £50 maximum, just choose something you like  

CPU cooler: You WILL want to be over-clocking that 2500k at some point   personally my cooler of choice is the Arctic Freezer 13 (non-pro-version). It's fairly small so there's no issues with height and spacing (92mm fan) and can easily handle overclocks of 4.6GHz. Solid price at ~£20 too.

Get an i5 3570k, new tech.

It's not really worth the extra cost from what I've seen so far. Keep hearing of lots of issues with over-clocking and heat due to the smaller 22nm set up.

2500K is rock solid. It's been proven time and time again on these forums and everywhere else. You don't get much better price vs. performance from any other CPU. Hence why the 2500k is so popular.

Its not worth the extra cost if you already have a SB, but for a completely new build you should get IB. It has way newer technology like PCI-E 3 which makes it futureproof, and is a much quicker CPU. Sure, it doesn't OC past 4.6 very well, but who needs that anyway.

Please ensure the following are compatible.

CPU: i5 2500k if you want to overclock. If you do not want to overclock, please choose i5 2570k

http://www.amazon.co.uk/Intel-Sandybridge-i5-2500K-Quad...

the following is recommended with i5 2500 as a CPU

MOBO: Asus P8Z77-V-LX. (others like the Pro version or ASROCK extreme iv are expensive...see the second thread)

http://www.overclock.co.uk/product/Asus-P8Z77-V-LX-Inte...

http://www.overclock.co.uk/product/Asrock-Z77-Extreme4-...

RAM DDR3 is cheap. Throw in an 8gb stick. I recommend G.Skills

http://www.amazon.co.uk/G-Skill-Ripjaws-240-pin-PC3-128...

GPU: GTX 560Ti. Ideally, GTX 680 or GTX 580 (they are very expensive)

There are many to choose from. I would probably prefer Asus or EVGA

http://www.overclock.co.uk/dept/NVIDIA-GTX-560--GTX-550...

COOLERS Many choices to choose from: Cooler Master Hyper plus or EVO, Cooler Master H100 or Noctua NH-D14

http://www.amazon.co.uk/COOLER-MASTER-RR-212E-16PK-R1-C...

http://www.overclock.co.uk/product/Noctua-NH-D14-Quiet-...

http://www.overclock.co.uk/product/Corsair-Hydro-H100-D...

PSU, CASE, DVD/BLURAY, HDD: plenty of choices from pretty cheap to very expensive. I would keep the CASE and DVD/BluRay costs minimum. I would get a good PSU. HDD is your choice...

Please see this review at Tom's

http://www.tomshardware.co.uk/ssd-storage-value,review-...

I did not recommend i7 2600k or i7 3930k, although they are very good, but expensive. i do not think i7 2600k is worth the extra money for performance. i7 3930k is fantastic but expensive (£450 plus).