Velcro_SP, google for N270 and northbridge. The nettop boxes don't use the N270 Atom. Plus the Northbridge 945GSE *when* running under full load is a powerhog. It sucks up to 22W. But Intel put powermangement to their chips. When on battery the clock is lowered and the powerconsumption is drastically reduced. Look, my Eee uses 10W when on battery, but about 25W when on 230V (battery not connected to avoid battery loading currents).
The 5121 does 760MIPS for its ppc core and is L2 less. It is a low end chip. Anyway, those 400MHz are sufficient for a lot of things and my Efika handles youtube better than the Eee. but this is, most likely, due to some weird JS/Flash container crap on the youtube site which Firefox is not able to do very good. But Tubexx just replays youtube vids fine. So it really is about what to achieve with teh maschine...
But the Eee handles *all* video partals, while the Efika only handles youtube. So, which system is superior now..?

Nobody yet has confirmed that the AXE core is speeding up things in real life applications. I guess it does, but still there are some cahche coherence issues reported which don't help to benefit from the AXE. Also note that the Atom comes with some MMX and SSE.

As for calculation: a netbook based on Atom and on a 5121 requires more or less the same parts. But where the Atom platform requires a chipset, the 5121 has everything on board. The 5121 is about 40 US $ IIRC (could bequite some less if ordered in real quantities), a N270+chipset is sold by Intel for 65 US$ (IIRC that is, I am too lazy to google around thera are other certain ppl who will provide links if I am wrong ).
The battery, screen, Ram, case are more or less the same compounds. If you assume you get a 5121 based device for 15- 20 % less in production and give those 15% to the customer, well. The Eee 900A is sold for about 300, maybe a 9" 5121 with decent battery will go for 250. The difference is not that huge.Also cost is not only cost of electronic parts, but also software support., If you build you netbook around some Intel reference design, you don't have to spend a single buck to get software support - it is already there. But if you chose some "obscure PPC", well....
Plus, the Atom *can* run Windows. It is very hard to strech the advantages of a 5121 based netbook. Still, I see some:
The power consumption is less and more constant (teh N270+chipset peaks at quite high values, the 5121 doesn't hav esuch a peak). A design can be cheaper, maybe the 5121 in conjunction with other really cheap, cheap compaounds could yield to a significantly lower price, which might attract ppl in not so developed countries (I guess that's teh approach THTF is trying to follow).
But still, competition by Intel is hard.