With regards to GPU's its also important to take account of your PSU. pretty much any motherboard will support a GPU but its down to the PSU fi your system can run it.
open the pc and check, I wouldn't think of upgrading/replacing a gpu without knowing what PSU I have and the power requirements of the gpu you are planning to use.
if you are looking at a 6670 then I think a 500w will be fine. How much budget do you have for a GPU upgrade, we might be able to advise something better.