It's not about the wattage when seeing if your PSU can handle the GPU it's about seeing if the 12v has enough amps. Keep in mind the 7870 is a DUAL PCI-E power card so if your PSU only has 1 PCI connector you'll have to go with the 7850. If you're only playing Guild Wars and MMO type games then a 7850 will be adequate paired with a strong CPU. The Phenom II should keep up for now but in ~2 years it's gonna struggle.
Edit: I see now your PSU is a CX500 well that will be plenty for a 7870 and it has 2 PCI connectors. So the question boils down to whether you're willing to pay $50 more for the 7870 when you're playing games that lean more towards the CPU? Still definitely not a waste, in 2 years from now MMO's will be taxing todays GPU's at a moderate level.
If you do opt for the 7870 I recommend the GIGABYTE version, it's quiet fairly cool factory overclocked by 10% and isn't voltage locked. Don't get the ASUST DirectCU II card it runs hotter with about the same noise level whilst also being voltage locked. And I think its factory overclock is less. As for the HAWK edition it's decent too but more expensive than the GIGABYTE version and it's cooler isn't a whole lot better.