Not being funny, 4-5 year old systems will still run WoW (maybe not great, but cheap and great don't normally mix). The pc I just replaced was a 4 year old athlon x2 running an x1600 and it did just fine.
If nothing else you can probably find a pre-built for 300-400 bucks. Just make sure it has anything other than onboard video. It'd cost you more than that to build it yourself even using bargain bin parts.