Today I received a mini ITX board using Celeron M ULV 373 and 855GM + ICH4 chipset.
I did some test of performance using Blender 3D to render a 3d scene, time in seconds lower is better.
Celeron M will not see any improvement for two Threads since it dose not have a HT as Atom or multicore as Q8400S so Celeron is slower but not by much.
Interesting that for the same clock Celeron M is 2x faster than Atom using one thread it was the same for pentium 4 maybe they use half the frequency internally and they sell this as high frequency as it will appeal to customers .
All system tested where basic configuration just MB with integrated graphic + CPU + RAM + SSD the difference is just that Atom also uses an 8.9" LCD since is a netbook and different power supply.
The Celeron M and Q8400 used the same power supply see photos
The power consumption was tested at idle in Ubuntu (Linux) and at load using Blender 3D all threads. The power was the complete system power measured at wall.
Celeron M 373 ULV
The Celeron M Board
The Q8400S Board
PS: The worst part is that the North and south bridge on all this systems is using more power than the CPU Q8400S idle 4W load 24W Atom idle 0.5W load 2.5W Celeron M TDP 5.5W (i have no other data about this one).