How can I best upgrade my system?

[Talone]

Hello, 2 summers back you helped me build my first pc for just over £600:
http://forums.overclockers.co.uk/showthread.php?t=18418378
PSU: OCZ ZS Series 550W
GPU: XFX HD 6870 1024M
RAM: Corsair XMS3 8GB (2x4GB) DDR3 1600MHZ
CPU: Intel Core i5-2500K 3.30GHz
Mobo: Gigabyte Z68AP-D3 Intel Z68
Case: Zalman Z9 Plus Tower Case
Optical Drive: Sony Optiarc AD-7280S
HD: Seagate Barracuda 7200.12 500GB SATA 6Gb/s

It does a great job for most games, but I'd like to be able to run demanding games (e.g. witcher 2/3, metro) at 1080p, high settings, high frame rates.
I haven't been keeping track of hardware developments, so I'd like your advice on how I can upgrade. Money isn't as much of an issue as it used to be - I can probably spend £400+.
Thanks for your time!

 

[Talone]

Thanks guys! Appreciate it. Over the next few weeks I'll get the 280x, an ssd and new power supply if mine is really that dodgy.
Why do I need that kind of wattage? is the gpu that power hungry or is it for future-proofing?

 

[Talone]

Ok, thanks. I may just stick with my PSU untill I want to do crossfire or whatever. Will consider the R9 290 also.