I'd go for AMD processors, they are fast and a lot cheaper then Intel. Also try to perhaps compare nVidia cards vs. HD Radeon cards, there's a lot to gain there too.
AMD FX-6100 6-Core Processor 3.3GHz = $119.99
Kingston Technology HyperX 8 GB 1600MHz = $36.99
Western Digital Caviar Blue 500 GB SATA III 7200 RPM 16 MB Cache = $64.18
HD Radeon 5750 1GB = $120.99 (Personally I'd throw in about $50 more for a way better videocard because I play new games, up to you.)
Case +/- $40
Other parts like coolers etc $75-$100?
You can also look into 2nd handed parts on Amazon, spares you out a lot of money if you find a good deal. Perhaps Craigslist is an option too to look around, it all depends on how much time you want to put into it but it's worth the money you can spare out.
With the computer prices described above you can do all your MMO's easily and a bunch of new games on great settings.
You'd only spend $500 (Roughly £300) with those parts above at the most!
