I actually use an AC powered fan on the out-side of the push-pull on my H80i combined with a noise baffle mounted on the back of the case. It has more torque against the air flow.
You can get them off amazon http://www.amazon.com/AC-Infinity-HS1238A-X-Standard-Cooling/dp/B009OWRMZ6
should be a 110CFM 47dB.
The reason I use an AC in the final position is they're much more heat resistant and they don't bog down under restriction anywhere near as much at 12v fans do. The average 45CFM 12v fan only manages about 28 to 30 when pushing into a heatsink and putting one on the out-flow side generally just makes it idle due to the lowered air density. If the computer is going to run at idle I just unplug the out-fan and let the inner one do the work. Even the toughened-up fans with the HXXi series cooling kits don't push their full listed CFM. The AC one will blow chip bags off the desk.
If you want to get really mean you can put one on the push side with a small spacer for spreading the flow but they tend to be really noisy on the intake side. Inside a case it gets to be a droning grey noise.