7950 or 7970 | r9 280 or r9 280x

Solution
I would consider buying a new PSU,since 500W just isn't enough.
650W is the recommended wattage for the R9 280X.
XFX Core 650W is just what you need.
All the best. :)

Depends on what you're willing to spend. If you're on a budget, get the 7950 and OC it. If not, get the 7970 and OC it too. Or, you could get a GTX 760 or 770 with the money you're using to get one of its AMD counterparts.
 
The 280X is a rebranded HD7970GHz edition.
The differences between those two are marginal and not noticable.
Same goes with the 7950 and the 280.
Personally,I would get the XFX R9 280X Double Dissipation.It's a great card and you won't regret buying it.
What mobo,CPU and PSU do you have?
 



EVGA SuperNOVA 750w B2 is cheaper than the XFX unit, if I remember correctly. The 550w unit costs like 70$. the EVGA unit also costs about 70$ but is better because of the higher wattage (and you get more connectors). It's made by Super Flower which is an EXCELLENT OEM.