this is all highly unscientific - I tested it by starting my 700MHz intel architecture ubutu linux machine and compared times roughly with ppc linux on the imac.
Boot, application load and desktop manager initialisation take roughly twice as long on the imac.
That said simple applications such as gedit, or running vi in a terminal window perform just about the same once they've got going. Open office writer, although slower to load is perfectly usable, even if some of the menu options are slower. With something like AbiWord (untested) it should be fine.
Not having got the internet out to the garage I couldn't meaningfully test firefox with something like google docs, but just opening it up gave the same slower load.
That said, and even though the disk is rattling like mad on the imac, it's a usable machine, and probably utterly adequate for visitors to use to check their email.
I'm quietly pleased with just how good performance is.