No idea what g.c means...
Both the Seasonic 520W and the Antec HCG-620 (Seasonic internals) will power a HD6950 2GB fine, but ultimately it depends on the full specification of the computer. A safer bet is to go with the Antec HCG-620 which will provide ample power regardless of your hardware specifications along that GPU.
Ah, think you mean Graphic card. FYI, its referred to as a graphics card, video card or GPU (graphics processing unit).
Whether the GPU is a good price or not depends on what it will be used for. What games do you intend to play with it? What resolution do you play at? And what is the rest of your hardware?
Reason for asking is playing at less than 1080p with a really old chipset would make its purchase pointless, so its important we establish what you want from it, and what its being added too.