Only a £200 budget? Very tricky O_o
And use the Windows 7 RC, assuming you can get a hold of it (Microsoft no longer offers downloads of it). Otherwise you'd have to go with a Linux distro, and I have no idea how well Guild Wars would work on Ubuntu or such - there's no way around this without adding an extra £60+ for Windows 7 (Home Premium is actually that cheap!).
Anyway I don't think you can do much better than that - are there no spare parts you have which you could put in this to save money?
I quickly put this build together on Scan because I like a challenge, does something like that look suitable?
Still only a single-core processor, but it's AM3 and better (and yes the mobo does support AM3).