Here's some photos of my lights and vent set-up. It's a little bigger than yours, but essentially the same thing. I'm sucking the air thru the cool tubes and out thru a scrubber.

I have two 400w in the cool tubes w/470cfm fan. The cool tubes are barely warm to the touch. Since you have a cooled fixture, heat shouldn't be such a big problem, although at some point you might want to consider a larger (more cfm) fan.

Would two 150's be better than one 400? Beats the hell out of me. Somewhere on this forum I remember a thread about the 150watt hps club or something like that. you might want to do some searching and see what you can find and ask the folks who are using 150's.
PharmaCan Reviewed by PharmaCan on . 150W Digital Ballast, worth it? So I'm hunting around on Ebay and I come across a 150 Watt HPS Digital Ballast. I know digital is the "way" to go as it's more efficient, cooler temps and the bulbs produce more lumens. Question is, for such a low power bulb, would it really make that much difference over your standard back yard mechanic ballast? eBay: Digital 150 Watt HPS Electronic Ballast Grow Light 150W (item 330091251497 end time Jun-21-07 10:11:30 PDT) Rating: 5