Seems that I have hitted the wall with my current system:
Antec 3700AMB (back fan grille removed)
Enermax Noisetaker 370 W (apfc)
Abit nf7-s (passively cooled)
Tbred 1.33-2Ghz + Sharkoon HPS1 + Nexus 80mm (guesstimate @ 9V)
512 Mb DDR400
Hercules Radeon 9600 (passively cooled)
Hitachi 7K250 120 GB
Antec case fan 120 mm (1600 rpm version @ 5 V ~25 cfm)
Have to admit that its already pretty quiet system and a huge improvement to the previous one. I guess expectations just grew bit too high along the road. Currently, the primary noise originates from the Antec case fan and its quite pleasant pure air flow wooshing. At close range its also possble to pin-point other noise sources: PSU (fans) and hard drive, but those will fade away. Main problem is what to do when that air flow is essential in aiding CPU cooling. New slower fan or disabling it al
together will just force to add noisier CPU cooler and possibly also PSU will speed up in stress. I'm also bit skeptical whether the fan change will make any improvements that can be really heard (currently no disturbing side noises), especially after trying Nexus 80 mm (its a good fan but doesn't have any better cooling/noise performance than other good have fans I have come up with).
All I can think myself are sort of trade offs, so I need suggestions how to remove air at least 25 cfm without any noise.
Low power mobile CPU upgrade or passive watercooling probably would both work, but I think larger upgrades have to wait a bit.