I would take a lot of things into consideration before jumping.
What type of bulb are you using in the 2 250 watt ballasts you have right now?
If you're using 10k ushios, or 6500k Iwasakis, then I'd just put another 250 in the middle
with the same bulb. The reason I say this is that the 250 watt Iwasakis at a minimum are more powerful than most of the 400 watters out there. The ushios are close, and the only bulbs out there that are more powerful than the above mentioned are basically 400 watt counterparts.
If you are using a more high kelvin bulb, then a 400 watt Ushio would be great in the middle.
Take into consideration also, the fact that a 400 watt WILL up the temp of the tank significantly.
It has a larger arctube in it, and the bulb is larger with more surface area than any 250 watt bulb, so it will radiate a lot more heat. Add fans is you decide to do this.
Also, I would give everything a good acclimation period to whatever bulb you add, as there will initially be a lot of unhappy guys in there if you don't.
Hope that helps...