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.