You can't actually compare the two in raw Mhz speeds.
Tell him to look at the latest Intel Centrino notebooks: They run at speeds around 1,5-1,7 Ghz, yet can still outperform some Pentium chips running at higher clockspeeds. Why?
And AMD actually gives their chips speeds in pentium equivalents, meaning a AMD Athlon 3200+ actually runs at a lower speed, but performs as fast as a Pentium 4 running at 3,2 Ghz.
So the speed (or mhz/ghz) a chip runs at is only 1 factor of how well a chip actually performs.
I can't give you an equivalent, but can tell you that other factors such as operating system efficiency and other factor that I don't know about play a bigger role than a few mhz more or less.
Also remember: The computer is only as fast as the software and the user. So if you take all day to finish that 1 page report, don't upgrade the system, motivate yourself!
If you can work efficiently with your iBook, don't bother comparing mhz speeds, it isn't worth it IMO.
If on the other hand you get fed up of waiting for apps to open, files to play etc. THEN (and only then) do you look at updating your system. (Will not happen to you, that iBook should be snappy enough!)
As for the harddrive: 18.78 GB isn't a huge amount of space, but you can order the 12" with larger drives, so which model you buy (12 or 14") doesn't really matter.
18.78 isn't tons, but should be enough for the time being. (You could install tons of apps, or hold about 4000 songs or all your photos.....
If you find yourself running out of space, buy an external harddrive and store things you don't need on a regular basis on there...
That's what I've done.