... but even knowing the minimum CFM rating it needs would be very helpful.
Well, I can tell you that 60 CFM is not quite enough.
Just wanted to share my experience with fan replacements. I put some Cougar Vortex HDB fans (
http://www.newegg.com/Product/Product.aspx?Item=N82E16835553005) in my cubes, and they sure do run nice and silent but at the expense of pushing a lot less air than the stock fans. The cubes do run on high with these fans (I ran them for over a day like that) but it gets way too hot for my liking. I touched a thermometer to one of the heatsinks and it read over 90°C (ambient is ~21°C). So I put the original fans back in and they're running much cooler now.
Whatever fan you decide on, make sure it has the standard three pins. Please share your experience.