Well you don't need to spend quite that much :lol:
This is the top-end of what I would spend:
PCPartPicker part list:
http://uk.pcpartpicker.com/p/12fIn
Price breakdown by merchant: http://uk.pcpartpicker.com/p/12fIn/by_merchant/
Benchmarks: http://uk.pcpartpicker.com/p/12fIn/benchmarks/
CPU: Intel Core i7-4770K 3.5GHz Quad-Core Processor (£235.99 @ Amazon UK)
Motherboard: Asus Maximus VI Hero ATX LGA1150 Motherboard (£151.91 @ Aria PC)
Total: £387.90
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2014-05-09 23:25 BST+0100)
Since you already have an h80i you should get a brilliant overclock out of it.
More realistically you don't need more than either of these options:
PCPartPicker part list:
http://uk.pcpartpicker.com/p/12hnl
Price breakdown by merchant: http://uk.pcpartpicker.com/p/12hnl/by_merchant/
Benchmarks: http://uk.pcpartpicker.com/p/12hnl/benchmarks/
CPU: Intel Core i5-4670K 3.4GHz Quad-Core Processor (£160.79 @ Aria PC)
Motherboard: Asus Maximus VI Hero ATX LGA1150 Motherboard (£151.91 @ Aria PC)
Total: £312.70
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2014-05-09 23:26 BST+0100)
Or this:
PCPartPicker part list:
http://uk.pcpartpicker.com/p/1Webc
Price breakdown by merchant: http://uk.pcpartpicker.com/p/1Webc/by_merchant/
Benchmarks: http://uk.pcpartpicker.com/p/1Webc/benchmarks/
CPU: Intel Xeon E3-1230 V3 3.3GHz Quad-Core Processor (£176.39 @ Aria PC)
Motherboard: ASRock Z87 Pro3 ATX LGA1150 Motherboard (£67.00 @ Amazon UK)
Total: £243.39
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2014-05-09 23:26 BST+0100)
The Xeon 1230v3 is essentially an i7 4770 (can't be overclocked, no IGP, slightly lower clockspeed).
In your position, I'd go and get the 4770k. And yes, it would make a big difference, though an i5 4670k would perform almost identically in the majority of cases.