Any idea on how much different power consumption is between 754, 939 and AM2? 2w, 5w, 10w? Varies depending on chipset?
Not a lot of data on that. I'd guess less than 10W.
All the K8 sockets (940/754/939/AM2/etc.) can use the exact same chipsets. The GeForce 6100 seems to be the coolest running nVidia chipset, and it is just as fast as the nForce4/5, and also the best overclocking chipset. VIA/ULI/ATI all seem to have cool runnning chipsets. Motherboard makers can choose what voltage to run the chipset at so power use isn't consistent anyway.
S754 is the most efficient since it has the least number of traces--for example you only use one big DIMM instead of two. Most people have figured out that SLI isn't worth the extra power usage, but yet still want dual-channel DDR/DDR2 even though the same power/performance applies there.
The only thing that needs tons of memory bandwidth is integrated 3D graphics. Otherwise single channel DDR400 is not a performance limitation.
As far as 1.4GHz K8. You need an additional 1GHz or so to even notice any performance difference. I wouldn't bother with the differences between 1.4GHz and 2.0GHz K8's. At most it is a 40% performance difference, and generally much less. For general use 1.4GHz K8 is very snappy.
The voltage required for stability increases pretty linearly with clockspeed, and power increases faster than V squared. So basically power increases in propotion with clockspeed squared
at minimum stable voltages. That pretty much means you want the lowest usable clockspeed for power efficiency.
The current K8 C'n'Q chips lock out core voltages under 1.1V which keeps them from having the best performance/power. The AM2 Semprons might be fairly good. The AM2 Sempron 3000+ does NOT have C'n'Q.
The Tforce 6100 lets you store sets of BIOS options, so you can have a 1.4GHz @ 0.88V option saved, and say a 2.1GHz @ 1.1V for when you want a little more power. Just requries are reboot. Not as convinient as C'n'Q, but usable.