Well I am planning to get a second copy of Witcher 2 but this one would be a retail boxed copy, coz I want to get the coin and the map. They still have some left in stock in online shops in my area.
Does this copy get the Enhanced Edition upgrade as well? How would I get the patches and stuff, coz my first copy is on steam so it did it by it self :(
Also how can I register my game? Is it even possible to register it on witcher site or gog?

Yes it will get the upgrade. The patches can be downloaded from either the official site or from GOG (read on)

http://en.thewitcher.com/the-witcher/2/

You will be able to register your game on GOG using the serial key found in the retail box
http://www.gog.com/en/page/witcherbackup

edit: corrected spelling mistake

I do not own a Steam copy of The Witcher 2.
Does the Steam version of TW2 come with serial key?

Thank you very much for a fast response that answered all of my questions :)

I don't own the Steam version either, but you should be able to verify your CD key. Look at the attached picture