Like everyone said, definitely don't go atom, because it'll cost more.
But just for the record, I think a 1.6GHz atom would be a tad faster or about equal to a 1.8GHz p4 celeron.
Here's my logic, and I know it's very approximate: a 1.6GHz atom is roughly equivalent to a 1.0GHz Pentium M Dothan acc. to AT. Now, a 1.6GHz Dothan is about equivalent to a 2.8GHz Pentium 4, acc to an old AT article on dothan. So if everything scaled perfectly with clockspeed, a 1GHz Dothan would perform like a 1.8GHz Pentium 4, and a 1GHz Dothan is also like an atom at 1.6GHz. Now I know nothing scales perfectly with clockspeed, but this is just an approximation.
So it seems Atom is roughly equal clock for clock with Pentium 4, and a touch faster than a p4 celeron clock for clock.