I'm sorry I do not remember anything at all about OS 10.1. (I own a 15" flat panel iMac but it's running 10.4, and About this Mac shows the processor speed.)
There were two flat panels - 15" came out first with a 700 Mhz processor, followed by a 17". This site might be able to help you narrow it down: http://www.apple-history.com/imac_flat
Turns out it is a mind blowing 40 MHz!
Something read wrong. The slowest FP iMac was 700mHz.
While in OSX, can you see if 10.1 has its own version of System Profiler? In newer OSX versions it's in Applications >Utilties.
I jumped from OS9 to 10.2 so, like Eustace, can't remember what 10.1 has--or doesn't have.
You can look dor the sticker/decal with the serial number. That usually gives the processor speed also: