I don't know you have the PSU for the new GPU or not. And I add the PSU +r9 280x, and the cpu will bottleneck the 280x. But you can use the 280x in the new pc if you want upgrade the cpu.
PCPartPicker part list:
http://uk.pcpartpicker.com/p/D698ZL
Price breakdown by merchant: http://uk.pcpartpicker.com/p/D698ZL/by_merchant/
Video Card: Sapphire Radeon R9 280X 3GB Vapor-X Video Card (£150.00 @ Kustom PCs)
Power Supply: XFX 550W 80+ Bronze Certified ATX Power Supply (£46.99 @ Amazon UK)
Total: £196.99
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2015-01-07 21:48 GMT+0000
Or if you don't want to upgrade the cpu, and get the r9 270 + PSU + 4GB RAM.
PCPartPicker part list:
http://uk.pcpartpicker.com/p/yxQFpg
Price breakdown by merchant: http://uk.pcpartpicker.com/p/yxQFpg/by_merchant/
Memory: Kingston Fury Red Series 4GB (1 x 4GB) DDR3-1600 Memory (£29.52 @ Amazon UK)
Video Card: XFX Radeon R9 270 2GB Double Dissipation Video Card (£109.52 @ Scan.co.uk)
Power Supply: XFX 550W 80+ Bronze Certified ATX Power Supply (£46.99 @ Amazon UK)
Total: £186.03
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2015-01-07 21:49 GMT+0000