Well I just bought a new PC for £750.
It has:
i5 2500k Overclocked to 4GHz
MSI Radeon R6850 Cyclone edition 1gb GPU
650W PSU
P8P67 LE Motherboard
8GB DDR3 RAM
1TB HDD
Case + Fans + CD Drive.
You could easily buy a cheaper processor and not get a sandybridge version.
Not get as much ram.
Not get as high a PSU
Get a smaller HDD
Added up around 500-600. It’s mainly the graphicss card you want to splash out on.
HTH
(Oh and I have been playing crysis and it is perfection.)