Right, so I configured a few builds.
Firstly, a Xeon build. Identical performance to an i7 4770.
PCPartPicker part list:
http://uk.pcpartpicker.com/p/3anpg
Price breakdown by merchant: http://uk.pcpartpicker.com/p/3anpg/by_merchant/
Benchmarks: http://uk.pcpartpicker.com/p/3anpg/benchmarks/
CPU: Intel Xeon E3-1230 V3 3.3GHz Quad-Core Processor (£185.88 @ Aria PC)
Motherboard: ASRock H87 Pro4 ATX LGA1150 Motherboard (£54.37 @ Amazon UK)
Memory: Corsair Vengeance 8GB (2 x 4GB) DDR3-1600 Memory (£64.39 @ CCL Computers)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive (£42.99 @ Amazon UK)
Video Card: Gigabyte Radeon R9 280X 3GB Video Card (£232.33 @ CCL Computers)
Case: Zalman Z11 ATX Mid Tower Case (£44.91 @ Amazon UK)
Power Supply: EVGA SuperNOVA 650W 80+ Gold Certified Fully-Modular ATX Power Supply (£64.99 @ Amazon UK)
Total: £689.86
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2014-03-15 20:06 GMT+0000)
Slightly over your budget.
Second build. I changed the CPU to an i5 4570.
PCPartPicker part list:
http://uk.pcpartpicker.com/p/3anru
Price breakdown by merchant: http://uk.pcpartpicker.com/p/3anru/by_merchant/
Benchmarks: http://uk.pcpartpicker.com/p/3anru/benchmarks/
CPU: Intel Core i5-4570 3.2GHz Quad-Core Processor (£137.99 @ Aria PC)
Motherboard: ASRock H87 Pro4 ATX LGA1150 Motherboard (£54.37 @ Amazon UK)
Memory: Corsair Vengeance 8GB (2 x 4GB) DDR3-1600 Memory (£64.39 @ CCL Computers)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive (£42.99 @ Amazon UK)
Video Card: Gigabyte Radeon R9 280X 3GB Video Card (£232.33 @ CCL Computers)
Case: Zalman Z11 ATX Mid Tower Case (£44.91 @ Amazon UK)
Power Supply: EVGA SuperNOVA 650W 80+ Gold Certified Fully-Modular ATX Power Supply (£64.99 @ Amazon UK)
Total: £641.97
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2014-03-15 20:07 GMT+0000)
Thirdly, an AMD build. The i5 4570 is likely to outperform this. Couldn't squeeze an aftermarket CPU cooler into the budget. You could chuck a Hyper 212 EVO in and that could get you to around 4.5GHz. It's likely that the 4570 would still outperform it most of the time.
PCPartPicker part list:
http://uk.pcpartpicker.com/p/3anwb
Price breakdown by merchant: http://uk.pcpartpicker.com/p/3anwb/by_merchant/
Benchmarks: http://uk.pcpartpicker.com/p/3anwb/benchmarks/
CPU: AMD FX-8320 3.5GHz 8-Core Processor (£104.39 @ Ebuyer)
Motherboard: Gigabyte GA-990FXA-UD3 ATX AM3+ Motherboard (£98.98 @ Amazon UK)
Memory: Corsair Vengeance 8GB (2 x 4GB) DDR3-1600 Memory (£64.39 @ CCL Computers)
Storage: Western Digital Caviar Blue 1TB 3.5" 7200RPM Internal Hard Drive (£42.99 @ Amazon UK)
Video Card: Gigabyte Radeon R9 280X 3GB Video Card (£232.33 @ CCL Computers)
Case: Zalman Z11 ATX Mid Tower Case (£44.91 @ Amazon UK)
Power Supply: EVGA SuperNOVA 650W 80+ Gold Certified Fully-Modular ATX Power Supply (£64.99 @ Amazon UK)
Total: £652.98
(Prices include shipping, taxes, and discounts when available.)
(Generated by PCPartPicker 2014-03-15 20:10 GMT+0000)
So there you have it. If you can stretch the extra ~£30 I'd grab the Xeon build in a heartbeat. If not, and gaming is the main concern, the 4570. Then there's always the 8320 if you're interested in a little worse gaming performance, but comes with overclocking capability and the 8 threads can be great for productivity apps such as video editing. The Xeon could take the lead in that department as well though.