Can I buy a Samsung Propel and use it as a Go Phone?

Yep. It's the same exact SIM card. Except i don't think Go Phone has 3G, which isn't a necessity.

I have a go phone, it's a piece of crap, and I want the Samsung Propel. So can I just go to an AT&T store and buy the Samsung Propel for it's full retail price and put my Go Phone SIM in it?

You can definitely insert your Go Phone SIM card in an AT&T network locked Samsung Propel phone. But make sure that the Go Phone SIM card is of AT&T network. If it is from some other network you need to unlock your Samsung Propel phone in prior. You can unlock your Samsung Propel phone safer and easier using unlock code. You can get the unique unlock code for a Samsung Propel phone from WickedUnlock.com at an affordable cost with an easy unlocking guide.