Unfortunately, you can't really do a wind tunnel on two of the bays because on of them you'll have to put your HD's in, and the cables will get in the way of being able to mount the extra fan (not sure how this works with SSD's but I'd imagine cables still present an issue). Then another bay will likely be filled by optical drives unless you are running external optical drives for some reason which will cause the same blockage.
What I did personally is to have my HD's in the top bay being cooled by the front fan in that bay, and that air blows back cooling the HD's and gets exhausted by the 200mm. The "Wind tunnel" is in the 2nd bay in front of the GPU (due to how the PCI-E x16 slots are situated you'll only be able to hit a single card with the tunnel at one time directly) so that it will flow across the GPU cooling it, and the the GPU fan's air will be caught by the tunnel's air and pushed out the either side exhaust or out the back vents.
Of course if you are real serious make sure to run them all on high (it's not that loud really since they are 120mm's). Also, make sure to route your cables as best you can to not restrict the air movement inside. Those can really make or break the airflow in your case, and a lot of people tend to ignore it or do a piss-poor job on it. Last but not least make sure to clean those fan grills frequently, and to take your case down and clean the crap out of it's insides with a compressor/can compressed air, and wipe the fan blades down with a rag as they collect an assload of dust on them that won't just blow off typically. It needs physical rubbing to remove it all (for me at least... damn things always full of dust :/).
Some 900 temps for reference for ya (and my setup is very amateur):
- Ambient temperature: 25c
- CPU - lolPentium D 2.8 Prescot (non-OC): ~15c idle, ~18-20c load
- GPU - eVGA 8800 GTS 320mb Superclocked (OC'd - GPU 500MHz>677MHz; RAM 400MHz>992MHz): ?? idle (hasn't idle'd yet since OC), ~63c load
I think your 900 will do just fine for you if you put all the fans in it you can. One thing I would be sure to do is measure to make sure you can fit the card(s) in your case. My 8800 is old school these days and it literally only has maybe 1/4 inch of space or less before it's touching the fan on the back of the "tunnel" bay. The 6-pin power cable on the back of the card is almost bent at a right angle where it had to be bent to clear it. If the new cards are any bigger than an 8800 you are gonna probably be doing some modding to get it to fit in a 900 (granted I have an original 900 so not sure if/how your "two" model differs in this area).