To be honest £600 is pushing it a bit, not impossible, but tricky. You do not say whether this £600 will need to cover operating system,monitor keyboard and mouse? Also apart from ArmA 2 mods what other games do you like to play?
Are you looking to future proof, though that's really a misnomer as it is pretty difficult to future proof a PC. But I assume you will be wanting the PC to be able to play games for a couple of years or so to come?
If you can provide answers to the above I will have a look about.