Here's a review you should look at.http://www.anandtech.com/systems/showdoc.aspx?i=3321&p=7
Atom is somewhere near an 800MHz to 1200MHz Celeron CPU in performance; The Celeron 220 should therefore be the faster of the two on average.
So is the Celeron 220 a lower voltage version of the Celeron 420?
Well..an ATOM cpu has atoms, and a celeron CPU has celery...which is also comprised of atoms, but since those atoms are busy BEING celery, the celery-on might actually be slower at the same saltiness.
interesting insight, I'll take that into consideration.... haha