How come my WoW runs slow since i've got my new graphic card " GT220 512 MB ?

So, before ive got my new graphic card , my computer freezed when it got to hot, and my ventilator was broken , so i bought a new graphic card yesterday a : GT 220 512 MB and now i see that when to much action occures on my screew when i play games it runs sooo sloww , and i don't understand why , ive installed all drivers