Nice build Jamie...
Just go for the 780ti here... they're end of life so going cheap - but be quick as they'll move fast.
http://www.overclockers.co.uk/productli ... subid=1402Alternativelt the new 970 is very VERY good value for money and the Maxwell chips overclock themselves with one button to ridiculous levels with VERY low power and heat. Amazing
You'll need a proper CPU cooler too. Recommend for your rig the Corsair 100i or 105 depending on whether you want software control of the fan and pump speed that you get on the 100i. Both very capable and will allow some room for the over-clock.
SJ