You might want to wait for Haswell refresh though. These will shift to a better TIM,which will help reduce temperature. Hence it will probably reduce the need for spending the better part of £100 for the cooler!
Avoid reference GTX760 cards as they throttle.
If you are looking at a compact fully modular 450W,this is nothing that really matches this:
http://www.scan.co.uk/products/450w-...0plus-gold-psu
However,it is very expensive.
They are now discontinued,so any remaining stock will be the last stock you can get.
Edit!!
An R9 280X will fit in smaller mini-ITX cases fine,so it will definitely be fine in a 250D.
You can get GTX780 and GTX780TI cards into pint sized Shuttle systems fine.
Regarding the CPU,there is this option:
http://www.scan.co.uk/products/intel...che-80w-retail
Lightly threaded games will see the CPU at 3.7GHZ and for multithreaded games,the HT will mean it will keep up with an overclocked Core i5. Plus you can use a cheaper motherboard and cooler too.
DX12 will see better threading,so I expect a locked Core i7 to keep up with an overclocked Core i5. For a game like Crysis3,HT shows a noticeable performance boost.