well first I'd start inside WoW.
go to the video options and check to make sure resolutions are the same (this way you are comparing apples to apples) and make sure your Vertical Sync is disabled.
then I'd do some kind of test, that is somewhat repeatable. An easy (but not especially accurate) test the the /timetest in WoW itself. In WoW, you can test frame rates, by going to a flight master and typing /timetest. Do the same flight path a couple times to get a good average, and then switch computers and do the same test again.
The /timetest won't really give you an accurate frame rate, but it will allow you to judge the relative performance of the 2 systems.
If you are still seeing a lesser GPU perform much better, then there's probably a problem somewhere... like drivers or something else. If the two systems perform nearly the same, then you'll know that your CPU is your bottleneck.
fwiw, you can upgrade that CPU without much difficulty... the AMD Phenom II X4 820 is the same socket and wattage, so it would drop right in ($90 USD on newegg atm).