Hey snap-one-off, reefsmoka - this was really useful thanks!
I just got a new PSU because the old one was causing sudden reboots, looked around a lot as I couldn't afford an enermax or antec and thought I might share my experience.
I had a 500W low-noise big-fan Q-Tec PSU but it recently started causing reboots every few minutes - it worked fine for a couple of months but I think my radeon 9800pro was stressing it, also I have an athlon barton XP 2500 o/c'd to 11*197. The PSU specs say "500W peak" which seems to mean you can draw that much for a few seconds before it conks out - the thing is really more like a 350W. It wasn't even especially quiet! I'm not normally taken in by crap like that, but it seemed cheap at 25 squid.
So I read snap-one-off's comment and went out to PC World to get a jeantech 450W - obviously usually I would NEVER go to pc world but it was a decent price (£35) for a change and this saved postage. Plus its easy to return;-) The jeantech is heavier, really can do 450W I think, has a decent 18A on the 12V rail, comes in interesting colours and has blue LEDs which get brighter the faster the 12cm temp-controlled fan goes. Up to about 35C (as hot as it gets in my case) it is really, really quiet - you can't hear it more than a couple of inches away. If you manually switch it to high, its still quieter than the q-tec at low.
So I immediately noticed that my coolermaster aero 7 cpu fan makes a buzzing noise which could be damped out by laying a finger on the plastic hood. I figured it was resonating so I stuck a blob of blu-tack on it - it's not pretty but it works, now the loudest thing is my hard disk
Anyway I also looked at the Nexus, which would have been 15 squid more but has good reviews on the web. What swung it was that the jeantech has active power factor correction which means it is a lot more more efficient (aparrently) and draws less power from the wall plug:
http://www.endpcnoise.com/cgi-bin/e/pfc.html
There's a slight overvoltage on the 12V rail (+0.4V which is within the 5% spec) but the others are all spot on. Its working fine so far anyway, I'm burning it in, will update if it turns out no good after all!
Hope someone else finds this useful.