The stock fan is already one of the best fans, replacing it will only drop it a couple degrees C, unless noise is an issue which it shouldn't be, just use the stock one.
Depends on how many RPMs the fans have. Go for the one with a higher RPM.
Actually, it's often not about the RPM's. Many fans with lower RPM's have higher CFM's and higher static pressure. It's about the design. A few RPM's doesn't hurt though. Heh. And actually, I just tried a Noctua NF-P12 on my 212 EVO heatsink and my temps actually went UP, although it was noticeably quieter.
Actually, it's often not about the RPM's. Many fans with lower RPM's have higher CFM's and higher static pressure. It's about the design. A few RPM's doesn't hurt though. Heh.