You go for dB when you want silence and CFM when you want performance.
Yate Loon Fans are IMO the best fans for good cooling performance and Silence when required with the use of a fan controller.
If you want silence the best 120mm fan you can opt for is the Noctua NF-S12-800 (http://www.noctua.at/main.php?show=p...ts_id=4&lng=en
) It doesn't move as much air as the NF-S12-1200 but it is marginally quieter in comparison.
Kitting your rig out with a bunch of Noctua fans is going to make it near enough silent, except for your Hard Disks, and PSU (if your adventuruous you might try modding the PSU with a Noctua fan). The most you can do about HDD noise is buy an enclosure.
If you want a truely silent PC for a Media Centre for example, my suggestion is to cut out mechnical Hard Disks all together and opt for an IDE>Compact Flash adaptor, buy a 4GB Compact Flash card and use that as a HD. Install something like Geexbox (http://geexbox.org/en/index.html
) onto it and you got a silent media centre PC.
4GB not enough storage? Use a file server or Network attached storage Caddy to house a large Hard Disk in another room!
I know your maybe not looking for a silent PC but I got carried away and don't want to waste this all by holding down backspace!