bit-tech has tested how cooling varies with the number and location of case fans:
The Big Air Cooling Investigation

Some interesting results, e.g. with only one fan the side mount gave the best cooling.

Tests like these are a bit dubious because, as bit-tech themselves wrote, "...what will work for one case may not work for another and different fans have different strengths and weaknesses too." and "...it’d be patently impossible for us to draw solid conclusion that are relevant to each and every one of you...". Nevertheless, systematic comparisons of different cooling configurations are always an interesting read. Hardware Secrets recently published these: Which is the Best Place to Install a Case Fan? - Part 1 and Part 2, it'll be interesting to see how their findings correspond to those in the bit-tech article.