400 MHz
Very good start. The G3 iMacs were made before the DVD optical drive became as ubiquitous as it is today. A DVD-ROM was the only version offered and was a very price upcharge in 2000. Your iMac may have only a CD-ROM or a CD-R drive.
Do that "About this Mac..." thing again and, this time, click on the "More Info..." button in the same window where you found your processor speed. That should bring up System Profiler. On the left side of the System Profiler pane, select "ATA" from the list. It should show two ATA buses. One should show info on your had drive and the other for your optical drive.
I can;t give you the exact format because out only Panther 10.3.9 Mac is currently lending its monitor to another computer, but the info for the optical drive should look something like this, from my G4 tower running Tiger:
HL-DT-ST RW/DVD GCC-4480B:
Capacity: 383.89 MB
Model: HL-DT-ST RW/DVD GCC-4480B
Revision: 1.03
Serial Number:
Removable Media: Yes
Detachable Drive: No
BSD Name: disk2
Protocol: ATAPI
Unit Number: 0
Socket Type: Internal
OS9 Drivers: No
S.M.A.R.T. status: Not Supported
From that, we should be able to figure out if you have a DVD or not.
