Unless you want to stock with only very high-light demanding corals, 400 watt MH would be excessive on your tank. IMO there is no need to pay for the extra electricity or deal with the added heat.
I have 400w HQI + 150 VHO actinic on a 65g and I REALLY like it. Like mentioned before, though, be careful of acclimation.. its a lot of light. A nice clip on fan from walmart takes care of my heat issues.
IMO dual 400s on a tank that size is overkill. You'll have enough light for just about anything under a pair of 250s in there provided you're using good reflectors.