I would find out your motherboard model and see whether it can handle a faster processor - there should be some upgrades out there readily available. (If it supports the E8400 or Q6600, either one would be enough for WoW).
The RAM is sufficient (assuming it's something like DDR2 800), but you can't use all of it with win7 32-bit. You shoule be able to install 64-bit from the same disc, aftrer backing up your personal files.
Also, a new graphics card might also help you out - the recently released Radeon HD 6850 is an excellent price/power point, but if you'll never go beyond WoW it may be more power than you need (a similar performance point for a slight increase in money is the Nvidia GTX 460 - I've heard people say WoW performs well with Nvidia cards, and if you can find one cheap this would serve you very well).
In either case, a 1Gb model would be advisable.
Really a video card update should get you playing WoW at max, it's not the most intensive CPU game anyway. A GTX460 or even one of the new 68xx cards from AMD will run WoW for at least this entire expansion and probably the next.
Although WoW will consume CPU, I'm not sure it's worth sinking over $150 into a CPU upgrade. I'd probably just upgrade the video card and save the rest to start working on a new build.
What's your budget?
For around $200, you should consider the nVidia 460 and the ATI 6850 or 6870. If your budget is higher, you might consider a 470.
I run max settings at 1920x1200 with a Phenom II X4 965 and an ATI 5870, and I get about 30-35fps in Dalaran at primetime. Usually about 50 fps in raids, though it can depend on the fight, sometimes it dips lower.