Case: AvP Defender 300 K1 Mid Tower Case £28.11
CPU: FX-6300 £78.40
RAM: G-Skill 8GB Ripjaws X DDR3 2133 £56.70
HDD: WD Caviar Blue 500GB SATAIII £37.99
PSU: Corsair Builder Series CX 500 £44.84
Motherboard: Asrock Socket AM3+ 970 Extreme4 £60.91
OS: Windows 7 £88.95 or Windows 8 £105.19
Optical Drive: LiteOn IHAS124-04 £11.80
GPU: GTX 760 EVGA £191.80
Total : £599.50 (windows 7)
This is a solid build that will be high/max on most games(i. e. Skyrim{modded}, BF3-4, Dues EX Human Rev, Civ 5, SC2{until there are 200 units on the board then it drops}) 60fps. Though if you need a monitor, mouse and keyboard feel free to trim the edges of this PC. You will also be OK with Minecraft, COD, and DayZ.
Monitor: Asus VS228DE 21.5 inch Widescreen 1080p is around £85.49
Mouse and keyboards can be found pretty easily.
Sources: Amazon UK