Ohh, that's something new to me. Didn't know γειά σού is also used as bye, thought just as "hello". xD
Well thanks for sharing that, I appreciate it. :]
You spelled them both correctly- the difference is basically a distinction of formality- "γεια σου" is used when speaking to a family member, friend, or someone you know well who is a similar age as you or younger, while "γεια σας" is used when speaking to someone you don't know well, is older, or in general someone whom you wish to show respect. Keep in mind that the greeting literally means "health to you" and so the "to you" part is the "σου" or "σας" I hope that helps.