Really, it's not so much about fan speed, although that is an important factor when choosing new case fans. A critical part in selecting a fan, in my opinion, is its static pressure. The more static pressure (measured in mm H2O) a fan has, the better it will be able to push air throughout your chassis.
As you know, cases are filled with things that obstruct the flow of air, such as cabling, videocards, CPU coolers, memory, errant wiring, etc. A high CFM fan does nothing if the air it is blowing cannot get past those obstructions. This is why a high static pressure fan makes a great investment because it is able to push air past those obstructions and deliver cool air to the most remote parts of your chassis.
I have had great success with Silverstone's AP series of fans
. They come in 120mm and 180mm flavors, and have a high static pressure rating. They also blow air in a column rather than a cone (see video in link). This allows air to be delivered with more force at longer distances than air being blown in a cone which loses force (dissapates) over a wider area. I had the opportunity to review a set of Silverstone's AP121 fans, you can read the review (link below) and get a better understanding of why these fans are so special: pcgamingcorner.com/wordpress/?p=2958
I would also look into a postive pressure system setup if you're going with fans for now. The Corsair 600T chassis' are excellent cases to do this with. Simply orient the top 200mm fan so that it is intaking air into the case instead of exhausting and you're good to go. Make sure you have good filters installed on your intake fans... I found that Kenmore HVAC filters from Sears make terrific filters when cut to size, and they are pretty inexpensive to boot.
Good luck and let us know how you make out!
<message edited by garetjax on Thursday, July 05, 2012 1:37 PM>