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Considering you could have got a 780 for a little more I'd say you did a mistake.
the classified was a week only deal from overclockers for £359 with splinter cell & metro last light
I had the chance to get a new 780 for £470 delivered msi TF but i opted out
but most 780s are 500 upwards which isnt close to the price I paid for the classy
The 4GB wont make a blind bit of difference, the limiting factor of those cards when upping the res is the smaller bus size, and even then it's not untill you start getting past 4m+ pixels does it really show its weakness.
Play some games on your setup, you should be able to play games with decent settings @playable FPS no probs.