There are many quality PSUs, but few are both powerful and silent. I have owned many Enermax units, and every Antec TruePower I have owned has run much quieter; the voltages were also much more stable. If a silent computing environment is what one is seeking, then the best choices (by FAR - even more so than the TruePower series) are the Nexus NX-3000 and the Zalman ZM300A-APF. Enermax would not even appear on my list if silence was of paramount importance. Many users and reputable reviewers (including the AnandTech guys) highly regard the Antec TruePower series for its performance and low noise levels. As for the so-called "silent" PC Power & Cooling PSUs, they are far from silent. While I agree that PCP&C's power supplies are probably the best power supplies for servers and dual-CPU workstations, they are simply too loud (and unnecessarily powerful) for the consumer market (the are also much costlier, and have not improved much since their earlier days - their reputation is what sells their products). There are 2 power supplies that suit your system well:
Zalman ZM300A-APF: One of the quietest PSU ever made, but maxes out at 300 watts, though a 400 watt version will soon hit the retail channel. Since you are running a 250 watt PSU right now, this should be more than enough for your current rig, though it probably lacks the juice for any future processor/HD/optical upgrades.
The Antec True430: While not quite as unobtrusive as the near-silent Zalman, it is still very quiet and will suit all but the pickiest of users. It also provides much more power and better voltage regulation than any PSU in its class (including Enermax). This power supply will not only sufficiently power your existing system, but it will also provide enough power for any future upgrades you may encounter.
Good luck with your system.
Drives I Own:
Asus S520/A, Plextor UltraPlex 40TSi, Lite-on XJ-HD166S, Pioneer DVD-106S, Toshiba SD-M1712, Plextor PX-R820Ti, Plextor PX-W1210TS, Plextor Premium, Samsung SM-352B, Teac CD-W540E, Yamaha CRW-F1, Plextor PX-708A, Sony DRU-510A