I have 3000 RPM Ultra Kaze's as Push, and AP-15's as Pull.
Ultra Kaze's at Lowest setting on NZXT Mesh Fan Controller @ ~ 6.5 V
AP-15's Direct Molex @ 12 V
This combination yields 2-3 C better temps then the original Corsair fans at full speed as push with some Slipstreams as Pull. But the new setup is quieter then the stock Athlon II X3 cooler in my server sitting 3 shelfs below my main rig.