I prefer to stack mp5 until around 380ish with water shield (while casting as that's the only one of those that matters) and then work on other things.. my priority list is roughly:
mp5 > haste > int > spellpower > crit
Once you're pushing 400 or so mp5 you can stop being obsessive about it, but the fact of the matter is that we don't have any talents or anything that improve our mana regen as a percentage of a stat (the way every other spellcasting class does) so we need mp5 and we need a lot of it. After you've taken care of the problem where you go OOM in every boss fight, you can start improving your actual heals. As someone noted above, Chain Heal (and Healing Wave) are 2.5 sec cast times with no haste, this is completely unacceptable so haste becomes our next priority. The nice part of this is that as you collect gear with haste and mp5 on it your spellpower will just sort of go up all by itself because it's on the same gear. You don't need to gear for spellpower (though consider gemming for it a little, the purple gem that gives sp/mp5 is a good choice), it will just kind of happen.
At the time of this posting I'm sitting at 394 mp5, 487 haste (little less than 15%), and 2212 spellpower. At the moment I'm looking for gear that improves my haste more, though it's getting harder and harder to find while trying to keep my mp5 roughly where it is