Why would you put the optical disk in the top? The 900 is a great case for putting the optical drive at the bottom and moving the HDD bays up to provide front-> rear airflow over the ram to the CPU. Then you can use the space under the optical drive to store the excess power supply cables where they dont affect airflow. One HDD per drive bay wont affect airflow that much.
I can vouch for this. This is exactly how I set it up in my 902. My two small gripes with it: 1. your dvdrom is a hassle to get to if your tower in on the floor, but since mine is on top of my desk... it doesn't concern me and 2. it looks less aesthetically pleasing IMO. Well, at least before I added my fan controller. Now it's all dandy and sexy again
But yeah, your 120mm front fan will be in a straight line towards your ram/cpu/heatsink. Also, you can opt to put another fan inside the HD cage using the bracket they give you.... and your videocard won't interfere with it, like it usually does if you put it in the middle.