Bugger.
The cookie's been set for so long I forgot about that. It's quite sucky how Apple now require you to register to get to their Knowledge Base. To cut a long story short, it involves taking out the whole motherboard to get at the SO-DIMM slot.
So it's a Revision B iMac by the sounds of it.
Anyway, so you don't need a login, there's a facility at the bottom of the page where I can email the article to you. Just let me know if you'd like me to do that. It has a few small diagrams, but I don't envisage it using a lot of bandwidth.
On and on and on and on,
and on and on and on goes John.