quieter fan replacement

My Aquagate Mini R80 came with a fan for the radiator that's quite loud, and I'd like to find a replacement. I know it's this fan because I used SpeedFan to turn it off, and it's so much quieter (SpeedFan's reporting some insane RPM amount for some other fan, too, over 1.3 mil, even though I have no control over the other two case fans).
I'd like to have a 120mm fan to help make it quieter while also getting more airflow, but the radiator only accommodates an 80mm, and I'm not interested in spending for the R120 (since the radiator is bigger and fits a 120mm) even though I've been told the R80 is a bit small for my CPU (X2 5000+).
The current fan is listed as having an airflow of about 53 RPM and noise level of around 37 dBA (both on max speed, which is what I have it as because I don't think turning it down is the best idea).
I'd like to get a fan that has about the same airflow but is quieter, but it seems the one I have is about the limit in terms of RPM/dBA ratio. Does anyone know of case fans that fit such requirements? Are there other alternatives to making this fan quieter other than turning it down?
As a reference, the other two case fans I have are Aspire and have about 30 RPM at about 25 dBA.

Thanks for any suggestions!

 

Yeah, I found some nice quiet fans (low 20s dBA or below) with the CFM in the 30s, but it seems that the fan that came with the R80 is a specialized fan or something because I can't find anything close to it.
It would be nice if I could rig it so I could fit a 120mm. It would fit on the case, but then I can't attach the radiator anywhere. Hmm...