Hi there!
Where you should go really depends on what kind of diving you want. That being said, you can't really go wrong with Gulen! Both marine life and the wrecks are fantastic, and they are on all kinds of depths
If you decide to go for Gulen, you definitely want to fly to Bergen and arrange some sort of transport from there, Ørjan at the divecenter will be helpfull with this (the actual divecenter is in the middle of nowhere...).
Other good options would be Narvik, especially if you are into wrecks, or Saltstraumen, especially if you are into marine life and currents
In general, the whole northern part of Norway offers great diving, and so does the west coast.
In and around oslo, there are also some decent diving to be done, but unfortunately, most of the really interesting wrecks are pretty deep... The marine life here is not something I would travel around the world for. Oslo is really about wrecks, but should you decide to come here, I'm sure I can be helpful with logistics.
Like the previous poster, I would also like to mention that Norway i march is as cold as it gets in diving, so don't even consider anything but a good drysuit with a good undergarment. The northern/nort-western part is actually a little warmer than the southern part that time of year due to the gulf stream.
You could also try the Norwegian forums:
http://www.dykkesiden.com/forum/index.php
http://www.dykkeprat.no/
Good luck either way
Bjørn