I know next to nothing about eMacs but I've got a feeling that, even if I did, I'd still be confused by your question!
All computers, including eMacs, have two types of memory storage. One type of storage is provided by the hard drive (which stores software, data, documents, images, etc). The other type is provided by the RAM (which handles data during processing). It's the RAM which someone is talking about when they ask how much 'memory' a computer has got.
So, your question has got me confused because the 'hard drive' bit tells me you want to upgrade the hard drive. (You can't 'add to it'. You either need to replace it or (if possible) add an additional drive). However, the 'memory' bit suggests that you want to increase the RAM. So which is it that you want to upgrade? Hard drive or RAM?
If it's the RAM which you want to increase, then you need to be looking for a pair of 512MB PC133 SDRAM. (That will take your memory up to 1GB). I've just Googled for that and found countless suppliers.
If it's the hard drive you want to replace, check out the Maxtor link (together with the other information) here:
http://www.lbodnar.dsl.pipex.com/eMac/eMac-upg rade.html
I hope that this goes some way towards answering your question but, as I said at the start, I'm a bit confused!!
Chris