I didn't make this clear wrt processor - this is a g3 (were there G4/400 imacs?)

No.

Does the above still apply?

Probably. I don't know what exact memory type they use, but any apple
dealer should be able to tell you. It'll want PC100 SODIMMS. Your apple
dealer can probably tell you more. Alternately, these people (among many others) have some info:

http://www.memoryx.net/imac1.html

SODIMMS are like normal memory, but smaller so that they'll fit in
smaller computers like the iMac.

Technically you're safest buying RAM from the apple dealer, so if
there's a problem you can force them to fix it. Hmm.. a quick google
search reveals that the G3 iMacs are a bit fussy - they wants PC66 or
PC100 SODIMMS and won't accept PC133. That's a pain, because PC100 is
hard to get now.

It's up to you, really. I'd buy standard RAM and install it myself - but
I'm a sysadmin and tech. If you'd prefer to pay the (large) premimum to
get Apple RAM, then cool - that's the safest way. I don't know why they
want $200 to install it for you though, it can't be that hard...

You may also want to have a look at this site, which covers installing
iMac memory:
http://www.djonmac.com/

BTW: The G5/G4/G3 is just the CPU - it's the motherboard chipset that
determines what memory format is used, and typically that's just an age
thing. Apple used to use weird, non-standard RAM but found it too
expensive to make - it was better to move to use the same RAM as the
rest of the industry and share the development cost.

Craig Ringer