Sadly you didn't show us your CPU and Motherboard. Either way, PCIe IS backwards compatible, you can put any graphic card (as long as your PSU can power it) and it will work.
However, as pointed, your CPU *could* bottleneck it, which is why people are asking you which one exactly you have. In any case any GPU upgrade will be worth it from that 9500gt, as suggested, I'd pick an HD 7770, which can be found for around $80, it will play WoW just fine.