The case I bought was a fairly cheap one, nothing at all special, and cost me £40ish, which is about the bottom end for a case with an AMD-approved 300w power supply. I then did a little untidy modifying, which consisted of sticking a 120mm Papst fan, which I happened to have lying around, in the front of the computer. It's pretty quiet, as all Papst fans are, and, while I don't know the figures for the number of CFM it shifts, I do know that it's pretty damn good. It'll be better than a lot of the fans you get pre-fitted in cases, and if you want quiet but cool then Papst is about your best option.
If you're going for a midi-tower case, bear in mind that, if there are 3x5.25" bays, and the case height and depth are under about 45cm, you're going to have trouble if you want to put a device the size of your average CD-RW or DVD drive in the bottom bay. I've used mine as an exhaust hole - the inlet blower is strong enough to pump plenty of air out there.
If you're looking for a really nice case, yet one that's also a lot cheaper than most of the other cases that have been mentioned, the Global Win 802 fits the bill - it has lots of thumbscrews to make installation easier, overclockers.co.uk sell that one.
If you're looking at making things _really_ quiet, look at
www.quietpc.com, of course
