ORIGINAL: 378
Computer fans don't like 24V. Trust me on that one, seen plenty a guy build a crazy gaming PC, overvolt his fans, and smoke them within just hours. Besides, you can get some that will pull 200CFM on 12V, although they'll sound like an EDF the whole time.
They make 24v small fans. I think they use 10-15 watts or something.. Radio Shack used to sell little fans for 12v or 24v. I wonder if they still carry them....