Alright, since I've seen this question a few times here's some technical notes if anyone is interested.
Fans operate by creating a pressure difference (analogous to a voltage in an electric circuit). Putting two fans in series will theoretically double the pressure increase whereas two fans in parallel will double the airflow (this is assuming ideal fans, and I'm not going to go into fan curves). This means that fans in series are more suited for enclosures with high flow resistance; so basically, if you can, put the fans in parallel.
And if you do put the fans in series...
As I mentioned, there is a theoretical doubling of pressure for fans in series. What will actually happen is the air leaving the blades of one fan will be at an angle (equal to the angle of attack), and the velocity will not be evenly distributed across the diameter. This is a problem since fans operate best (sort of) with a uniform pressure/velocity profile at the inlet, and to make things worse, if the fans have equal angles of attack and are rotating in the same direction, the air will essentially become very turbulent giving rise to high viscous dissipation (ie power not going to moving the air). If someone cared enough to do it, it would be possible to put a stator in between the fans that would redirect the air onto the blades of the second fan (this is the same thing that is done in pressure-staged power plant turbines, just in the opposite direction). Flow straighteners would also add some benefit, so having the heat sink in between would help.
