Windows 7 Home Premium 64bit
Intel Core i5 2500K (with Arctic Cooling CPU fan for overclocking)
8GB RAM DDR3
NVIDIA GeForce GTX 560 Ti
Seagate 500GB HDD 3GB/s 7200RPM
(2 internal fans for case cooling)
Theres quite a few with near identical spec to yours on fleabay averaging £380-£470 but thats brand new. So based on that, I would assume you'd get between £280his -£325 as a used model. If it comes with a monitor, and usual peripherals then maybe you'll get £340 . . .