If you put the code in it will just add 13 months to your expiry date on your membership. Not sure if it will work on American accounts.
GOLD member privleges? The only things I can see different to when I 1st joined are the price reduced extra content packs, although most people will have brought them for the specific games when the the content was first were released so the reduction is pointless. It's only ever older game content that's reduced anyway!!!
Anyway LIVE is still the best online gaming round (and I've got a PS3 and 360 so can compare them), so get on it!!!