Linus regarding your CPU, noticed you are running an Athlon XP which in my experience is actually quite inefficient at idle. I believe the halt state/disconnect is not properly implemented for all socket A Athlons, if you run a software utility like S2K Disconnect, this will dramatically lower idle temps by up to 8C or more for a higher clocked chip (and assume power draw respectively). However this doesn't seem to work with all mbs and also poses stability problems even for mbs where it seems to work, probably why AMD was never able to implement it. In contrast the new A64/Semprons have significant power consumption drops at idle even without CnQ to step down voltage/clocks. I have read various articles/posts scattered through the net long ago, but sorry I don't have any links. Of course though if you're underclocking to 300 mhz and assume undervolting as well then CPU power consumption maybe already so low there is not a big difference.
In regards to the mb, I suspect that in systems like the Pundit, MacMini, and laptops, where the mb has to be designed specifically to work off the single voltage power input and have all the DC-to-DC components integrated, probably do so more efficiently than any mb designed to be powered with a regular ATX connector. The Pundi mb design is actually very clean and doesn't have a lot of components on it.
The efficiency of the DC-DC adaptors like the PicoPSU and PW200M are actually quite high. The new Pico manual (pdf)
now has specific numbers listed (both rails are rated Max=6A and Peak=8A). From SPCR's power dist article, the AMD systems drew around 2-4A on these rails.
DRAW - 5V RAIL - 3.3V RAIL
1A - 86% - 85%
3A - 94% - 93%
5A - 96% - 94%
8A - 93% - 91%
For the 12V rail, I believe basically it passes through the input 12V so efficiency should be close to 100% and as we know this rail supplies the majority of the power consumed in modern systems. In my experience the PW200M is even slightly more efficient than the PicoPSU. Of course this is just the DC-DC side, the efficiency of the AC-DC brick also needs to be factored in.
When I replaced the stock AOpen XCCube 220W PSU in my Socket A system with the PicoPSU I saw a dramatic 18-20W AC power draw reduction (Stock PSU = 70-85W, PicoPSU = 52-65W) and I suspect if you are using the flex 220W that came with your case it probably is performing at similarly poor efficiency. It is possible a Seasonic PSU maybe able to match or come close to the Pico, but I have my doubts. When I finally get back my S12-430 I will try to do some comparisons, or ideally if SPCR could put up a review that would be even better.
As for Josti's Lex PW004, I do suspect it has a lower efficiency than the Pico/PW200M as it works off a higher 17.2v input meaning in addition to converting the 5v/3.3v rails it also needs to convert the 12v rail.
As for the power draws I posted for my systems, most of them are just under Prime95 and not max draw. I'll try to run RTHDRBL and CPUBURN as well as loading the HDD/DVD and post up some figures later.