Hey Beckie, I went through the same sort of argument with myself, bought something with less than optimal lighting then got frustrated with all the cool stuff I saw that I couldn't have (including SPS and clams) so finally scrapped the whole thing and bought another system. So... I would definately lean toward "overkill" if I were you so you don't regret it later! That is, of course, if you can talk your SO into it I currently run 2X400 MH plus VHO actinic supplementation on a 48x24x24 tank.
Good luck!