You don't need special undervolting power figures. The rule of thumb is: the lower the frequency and voltage, the lower the figures.
: have a look at http://forums.silentpcreview.com/viewtopic.php?t=20906
. Conclusion: 2,8Ghz@1.2V
: have a look at http://will.dashwood.me.uk/pentium4m-2.htm
You need some FSB-changing tool. The voltage stays the same, though. For a tool, have a look at http://forums.silentpcreview.com/viewtopic.php?t=20872
. Conclusion: 1,2Ghz@default vcore
: All Athlon 64 (and all Semprons >=1,8Ghz
) support Cool'n'Quiet. Get the 90nm parts. Conclusion: 800Mhz@1,1V
: There is support for EIST which equals performance-wise to Cool'n'Quiet.
AOpen i855GMEm-LFS and DFI 855GME-MGF: Unstable support of EIST (http://www.silentpcreview.com/article218-page9.html
AOpen EY855-II: at least manual ChrystalCPUID support (http://www.silentpcreview.com/article236-page6.html
Aopen i915GMm-HFS: the german magazine C't claims that RmClock is unstable with the included Aopen fan speed tool. Which indicates that it is stable without the tool.
Overall Conclusion: P4 and P4M don't have very good sulutions. P-M EIST support seems to be quite rare.
P-M platforms are quite expensive, too. Further research would be necessary. The Athlon 64/Sempron provide a quite mature solution. But don't get a nForce 4 chipset.
With working EIST/Cool'n'Quiet a P-M and a Athlon 64 will need equal power at idle. The P-M platform should draw less power since it is optimized for the lower load power.