It has a 500w power supply. Whether it is an average, good or bad power supply (PSU) is in question though. Assuming it is of average quality you should be able to install up to Radeon HD 7870 or nVidia GTX 670 if the PSU has at least 25 amps (25A) on the +12v rail(s). You need to open the side door and read the stick on the PSU that supplies that information.
You can also take a picture and post it .