Pob the current X3's perform all right (the is one in that review so you can get an idea of how they perform) but they suffer from the same low clocks (and poor overclocking 3GHz is pushing them) that the X4s do the new Phenom 2 X3 (700 series, AM3) will be along soon they will come clocked faster and will probably be very good value for money at a guess the 720 BE (2.8GHz) will come in ~£120 and will have plenty of headroom for overclocking
http://www.techpowerup.com/84225/Phe...on_Tested.html in that little mini teaser of a review they had one up at 3.7GHz which puts it well into the any more speed really is diminishing returns for everyday use bracket.
As to a KillerNIC your having a laugh aren't you they are a waste of money when your running on a wired network at a LAN let alone when you have an ADSL or Cable Internet connection to contend with.
Finally when somebody says I have £2.5K to spend on a computer people tend to look big rather than sensible yes a low cost duel with a midrange graphics will obliterate WoW but its always nice to have that little bit extra available for when you decide you want to play Crysis or whatever.
Right so trying to keep to the brief this time
a system to run WoW well 2.5K to play with and a really nice monitor (missed that bit completely first time round)
CPU: AMD X2 7750 BE - £65
MB: DFI DK 790FXB-M2RSH - £130
RAM: 4GB DDR2-1066 5-5-5-15 - £60
HDD: Western Digital 640GB AAKS - £55
Optical Drive: DVD-RW Samsung or LG or Whoever you like - £15
GPU: ATI HD4870 1GB - £190 (bare with me on this)
PSU: Enermax Modu82+ 525W - £85
CPU Cooler: Scythe Ninja 2 or Thermalright Ultima90 (+fan) - £30
Case: Whatever you like I would look to spend £60+ and be looking at brands like Lian-Li and Silverstone both very High quality.
Monitor: Hazro HZ30W 30" - £690 - S-IPS screen 2560*1600 resolution
Homepage Decent review @ TFTCentral
Total Cost: £1400 (Included £80 for a Case)
There we go need to add an OS (or you could use Windows 7 beta I guess or Linux)
Other options:
Move to a µATX motherboard (perhaps DFI JR 790GX-M2RS £115) and get a smaller case.
Quad core processor Phenom 2 940BE £190 (but as people have said its power will be wasted running WoW)