I am buying an antec 900 case for my new build. I am planning on doing some overclocking, so cooling is very important. I was wondering what fan configurations people have used. I've done a little reading, and I can't seem to find anyone who has installed the two optional fans. Has anyone used all six fans and did it cool better or worse than using the four fans?

A reason that you may be having trouble finding anyone who has installed two extra fans is that the stock four fans do such a good job as it is. I use an Antec 900 with an overclocked QX9650 and everything is kept nice and cool, so there has been no reason to change anything. Like the old saying goes, "If it ain't broke, don't fix it".

I have a buddy that has the 900 case...that thing is like an air conditioner inside, there is so much airflow.

I use this case and do have all fans installed, all run at low speed and this thing stays cool. My former case gave me video card heating problems.
 
Running SLI setup with 2 8800 GT's
 
The air out of the top fan is cool at idle and warm under full load.
 
The guy at my computer store told me if this case is not going to keep your system cool you need to go with water cooling. It has ports for that as well if one needs it.
 

I'm using this in my new system.  I'm keeping the front and rear fans on low, and the top fan on medium.  I bought optional side and mid-case fans, which are on the equivalent of the Antec fan low.  Seems to be working out nicely.

The two front fans intake, one rear exhaust, top exhaust, side panel optional exhaust, middle two optional blowing to the video card...