As long as you do not need a new monitor and the other usual junk or a SLI/crossfire system £700 is more than enough to net you a decent games machine.

If you are around here then I assume software is not a problem so it might be worth looking at a barebones system- careful if you do though as barebones has become somewhat of a catch all term in the last few years and some still use the original (proper?) meaning of case, motherboard, PSU, CPU and ram- you sort disc drives, hard drives and everything else while others mean you get a box that will plug in and go- you sort operating system, monitor(s), mouse and keyboard.
http://www.overclockers.co.uk/ have a fairly decent example of the latter sort of thing.

I will ask though are you planning to salvage anything; we have all seen motherboards take out everything but there is still a chance some stuff works?