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is this rig decent

You could go to intel with this build:
PCPartPicker part list / Price breakdown by merchant

CPU: Intel Core i3-4150 3.5GHz Dual-Core Processor (£84.56 @ Amazon UK)
Motherboard: MSI H81M-E34 Micro ATX LGA1150 Motherboard (£35.28 @ CCL Computers)
Memory: Patriot Signature 8GB (1 x 8GB) DDR3-1600 Memory (£40.64 @ Ebuyer)
Storage: Seagate Barracuda 2TB 3.5" 7200RPM Internal Hard Drive (£53.94 @ Aria PC)
Video Card: XFX Radeon R9 280X 3GB Double Dissipation Video Card (£158.81 @ Scan.co.uk)
Case: Cooler Master N200 MicroATX Mid Tower Case (£30.37 @ Amazon UK)
Power Supply: EVGA 500W 80+ Bronze Certified ATX Power Supply (£39.00 @ Scan.co.uk)
Operating System: Microsoft Windows 8.1 OEM (64-bit) (£75.34 @ CCL Computers)
Total: £517.94
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2015-06-13 13:20 BST+0100

( £50 left for peripherals)
but your AMD solution looks good enough - Windows 7 and 8 will both get the upgrade to 10 for free so just go for whichever is cheaper 😀
 
Solution
Comments on your build.
1) Only get the fx build if you get an aftermarket cooler and overclock the fx-6300. If you don't then get a i3. The i3 will beat a non overclocked fx-6300.
2) 500W is on the low side. Get at least 550W to be safe. And 600W gives you more freedom in GPU choice.
3) Which Windows version? Get whatever is cheaper and upgrade to Windows 10 in July.