Steph is definitely right. There are plenty of options that you can turn down to keep your frame rate up. The best part is that you can get lower end 6000 series nVidia cards for relatively cheap, and with those you can run WoW without any problems.

If you make sure you're not doing any anti-aliasing, turn down your texture settings and draw distances you can run WoW without any problems.

My main box is an Athlon 2200+ (almost 5 years old now) with a GeForce 6500 and I'm running 1280x1024 with standard graphic options and getting 30fps in most places. I paid about $1000 for it back then, and I know you can get even more nowadays for less than that.