I have been doing some research and this is what I have come up with for around £600:
Motherboard: Gigabyte GA-Z77-D3H USB 3.0 Intel Z77 Socket 1155 Ivybridge Ready Motherboard with USB3 =£78.92
CPU: Intel CPU Core i5 Unlocked 2500K Sandy Bridge Quad Core Processor=£166.98 (I had also considered the 3570k for £178.98 or the i7 3820 though I would have to upgrade my motherboard to one with 2011 support, would either of these better for a gaming rig and if so why?)
GPU:HIS HD 7870 IceQ 2048MB GDDR5 PCI-Express Graphics Card (H787Q2G2M) =£199.99 (with Sleeping Dogs and Nexuiz included)(I had also considered a SAPPHIRE RADEON HD 7950 FLEX OC BOOST 3GB GDDR5 for £230.87, would the extra money be worth the performance increase? Benchmarks I've checked say they are quite similar.)
Power Supply:Corsair Enthusiast TX V2 CMPSU-650TXV2UK 650W Power Supply (PSU) =£70.38 (For the components I've listed would this be enough or would I need more?)
RAM:Corsair Memory Vengeance Low Profile Black 16GB DDR3 1600 MHz CAS 9 XMP Quad Channel =£60.14
Everything else (case included) will be salvaged from an old Dell Dimension 5100. As a first time builder I was wondering if anyone could be kind enough to tell me if these components would work together (would I require a fan if I wanted to do any overclocking?) or even if they are worth getting (are their any better ones with similar if not better performance for less money?) Also, if anyone could tell me how future proof this rig would be I would be very grateful.
I would not consider the i7. It will make very little difference in gaming.
The 650w is a little bit overkill. Get a 550w-600w.
16GB is overkill as well. Get 8GB, and make sure it is Dual-channel, not Quad-channel.
I would not use that Dell case. Cable management will be a nightmare. Spend 25 pounds and get a case, like the Cooler Master 330. If you want to overclock, an aftermarket cooler is imperative. The Hyper 212 EVO is a really good cheap cooler.
Also, there is no such thing as futureproof. Every computer is destined to get less and less power over time.
Thank you for the help, what I meant by future proof (and it was my fault for not specifying it, sorry) was how long it would be before I would have to upgrade my computer again (i.e would it be worth spending a little bit more on any of the components in order to ensure that it is longer before I have to upgrade again or will I be fine for a while with my current options)