xlo, decibel measurements are on a logarithmic scale, not linear. I think (someone correct me if I'm wrong) that a 10dB increase means 2x the loudness, so that 27-37dB difference makes the orb TWICE as loud. Or something like that...
Now on topic...
I've built relatively quiet systems for myself and my GF.
My GF's system is a P4 1.6 (not Northwood) with the retail-box HSF and a Leadman 300W dual-fan power supply (manufacturer claims it is quiet). No case fans, no Northbridge fan and the video card is a Geforce2 MX with a heatsink only. System runs cool and quiet. The loudest part of the system is her IBM Deskstar GXP 75 hard drive.
My system is a Celeron 1.0A overclocked to 1.33 using the retail-box HSF, Eagle power supply (loudest fan in system) and an Enermax adjustable fan set to medium-ish. The video card is an eVGA Geforce2 GTS 64MB with a huge passive heatsink replacing the normal little hsf. No Northbridge fan either. I have three hard drives, all Maxtor/Quantum 540X series 5400RPM drives. Two 60GB on RAID 0 and a single 40GB as drive C:. I'm going to replace the single drive with a single 80GB Maxtor "L" series 740X 7200RPM drive (fluid bearings). According to Maxtor, the noise output at idle of the 540X is 30dB while the "L" series 740X is 27dB.
I feel that these systems give a good compromise in terms of relatively low cost, ease of assembly (no fancy modifications), decent performance and low noise. I do know that both these systems outperformed the ones they replaced while being NOTICEABLY quieter.