As Jim said, over-volting fans is not recommended. For a good bit more cfm for not a lot of pennies, i'd recommend Xilence as they push (on paper!

) 68cfm at 1400rpm as are very quiet. Excellent fans for thermochill rad's. If using the like's of black ice rad's, use the same or go for fans that run faster (2000rpm) which should get you about 80-90 cfm and still not making your ears bleed. i.e, yate loons, silenx etc...
You only need a case thats big enough to house the rads you want. Lots of variables here as it will depend on how many loops, what you're cooling, what you can afford etc etc. You can also mount rad's externally, i.e, on the back of the case, or you can get a custom made external box for house up to a 120.3 from some dude off the net. Can't remember the site but i'll find out if needed. Only prob with that is you'll end up using onger lengths of hose...