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http://www.homedepot.com/h_d1/N-5yc1vZ1xgt/R-100081599/h_d2/ProductDisplay?langId=-1&storeId=10051&catalogId=10053 $14 bathroom fan @ 50 CFM. Comes in a lightproof housing. I would trust the bathroom fan's ratings over the computer fan's (especially under load).

That mercury vapor isn't doing anything but adding heat. I would switch it out for some more CFL's or those T5's. The main thing is your plants look healthy. If you can get your hands on an HPS, though, it will pay for itself.