I just recently have a chance to test with various system running an Interbase database server. Mostly older system, here's what I found out:
Among the system I tested -
2.8Ghz P4 northwood core is the fastest.
1.8Ghz Pentium D, pretty close, can't really tell, but then it has more ram still not going faster so I give it 2nd

2.2Ghz Celeron, new one, I assume it's using core architecture, probably as fast as the pentium D.
1.6Ghz Celeron, don't know which one, but almost 2x slower than the 2.8Ghz Northwood.
1.2Ghz Duron Morgan core, a little slower than 1.6.
And lastly...
Atom 1.6Ghz, it's like 5-10 time slower than the 1.6Ghz Celeron, even with more ram.
Problem is, the cpu isn't even running, cpu time is at 5%~ but nothing else moves. I can only think of it as in order processing vs out of order processing...
Well, people will say you don't need more for surfing web, streaming youtube, running office application, basic multimedia etc etc. But then after setting up 175 Atom powered pc for the pass 3 weeks, I'd say the wall is very near for you to hit... er.. whatever.
I think for cheap(er) portable processing power, yukon platform might just hit the right spot (since celeron mobile use more power and ULV models is, well, slow, with nothing in between to close the gap).