Here's a screenshot of Speedometer 4.0 results from a slow-by-today's standards workstation. (A first-edition 2006 Mac Pro with the downgraded 2.0 Ghz CPUs, running 64 bit Ubuntu. SheepShaver's running MacOS 7.5.5, which I realize is a little weird. The results with 9.0.4 are similar, however.) According to this:

http://lowendmac.com/benchmarks/speedo4.shtml

They make it about as fast as 400Mhz G4 Powerbook. Thus the off-the-cuff "G3 ballpark" estimate for an Atom.