Ahah. My area of expertise.
Basically you have two options:
1. Keep your DDR2 and buy a second-hand 775 pin Core 2 Quad processor and motherboard. DDR2 motherboards are mostly end-of-line and not worth buying new. You could pick up a C2Q like a Q9550 and S775 P45 chipset motherboard for very little cash. I prefer Gigabyte motherboards but anything from Asus, MSI or even Asrock should be fine. All you need to do is add a decent budget graphics card and you'll be able to run WoW with ease. For the graphics card, go for an ATI Radeon 5770 or nVidia GTX 460 (grab the 1gb version). You should be able to find either second-hand for not much.
2. Ditch the RAM and go with a new budget-build. I'd suggest an Intel Core i5-650 paired with something like a Gigabyte G-B P55A-UD3 motherboard and 4gb of DDR3 1333 RAM (G.Skill or Kinston value will do the job). Grab one of the video cards mentioned in point # 1 above.
There are a couple of other points to consider - if you are upgrading from an AGP system, you might need to buy a new power supply. Old, cheap power supplies are dangerous and frequently die, taking out the rest of the system with them. Do you know what power supply you have at the moment? If not, have a look on the side and post the brand and model. Depending on what you have, you might want to pickup a quality budget PSU like a Corsair VX550w.
The second consideration is overclocking. If you build either of the two systems above, you'll be able to overclock the processor and/or GPU to get much better performance out of it. If you use the standard Intel CPU cooler you should be able to get at least 20% more performance but if you invest in a decent cooler, you could get much further. For example, I have an Intel Core I7 920 which runs at a default speed of 2.66ghz but I've been able to overclock it safely to 4.0ghz. A good cooler (I use a Noctua NHU12P-SE) is essential to this and will obviously add to the cost of your build.
The bottom line is that either build should have no problems running WoW - overclocked or not, so overclocking is not a requirement.
A couple more questions
1. What resolution do you play the game at? If you don't know, how big is your monitor (in inches)?
2. Were you planning on building it yourself (very easy) or having a store to put it together for you? You'll save a fair bit by building it yourself and learn a useful skill. If you decide to do this, there are plenty of guides on the net. As I said, it's easy; every part just slots in where it should so if you have to force something, you're doing it wrong.
*edit* Just read that you already have a PCI-e GPU. What card is it? Depending on how new it is, it may or may not be good enough. Post up the model number and I'll let you know if it will be up to the job.
