Well, if you're running CFLs, you could get some 120 mm fans that are 28db (quiet) and under with about 50CFM each from a computer store for cheap ($20 at most, check actual computer stores and not Office Depot or the like). Also, an oscillating fan would be great for plants as it builds stem strength, keeps temperature in the box down and keeps air from getting stagnant (which can prevent certain times of fungi and infections or something (I've never grow marijuana, so I can't say exactly, but I've read it's much the same as growing tomatoes and what not indoors, the growFAQs here are great (even if you don't grow cannabis, they help make the nicest grow boxes ever (much better than some commercial ones))).