You must have your own original copy so you can register the serial key to your forums/Impulse account. Used copies that have already been registered by the original owner cannot currently be transferred.

You can register within the Impulse app itself if your friend did not do so himself. There's a guide to registering available here.

Yes, you can purchase the game direct from us on impulsedriven.com.

Mr9, please send me a plane ticket + per diem + room & board accomodations (for 4 weeks) and i'll bring you a copy of demigod (and 10 other games of your choice, plus a new computer).

In answer to your question though both will give you the serial key.  The retail box will be mailed to you but you will still get your key immediately.

j