First question... how much are you looking to spend ? I msure you will get some money selling off you old PC as a whole.... or do you want to use some of the bits ?
Even as a temporary upgrade i'm looking to replace/upgrade parts in my computer that will give me the biggest FPS increase, which is all im concerned about currently.
As far as biggest FPS will go eith upgrading your current PC, The Gpu to 3850, replace the power supply to handle the 3850 and get 2gb more Ram, total cost roughly £160-£180, Personally far too much to go on an old system, hunt around on e-bay for second hand parts or look in micromart classifieds...
And then you are not talking a huge framerate increase here at all, in large raids in wow, you will still get performance issures, as the cpu is the major bottleneck as well, you could get a faster one, but then you have built a new OLD pc so might as well have built a New NEW one.....