what servers can you transfer from/to?

Generally.
Nexon can be arbitrary though.
Yellonde or Demethos took forever to be unlocked if I recall. Newer ones have been much quicker.

oh yea, that seems to bring back memories of people complaining... i wonder if they'll hurry up after doing a "get to 30" event... geez, so annoying, allright, thanks a lot for the answers!