Grow Tent Lighting??

I have a tent that is 3'X3'X7' made from PVC and PandaFilm. In each corner hangs a pair of 48" Fluoros. I will be hanging an HID from the top however I am worried about heat problems. I will cool the light using cool tube and a 4" fan (125-170cfm) pulling air from the tent itself goin into the cool tube and then exhausted.
My question is should i use a 250w or 400w bulb and ballast?
Also I Will Be Using MH-Veg and HPS-Flwr.

Thanks In Advance...

400w will work great but make sure you cooltube it. PVC tents can get very hot and when they do they release harmful chemicals into the air