Its specs list 1 GB as the maximum. I don't have any experience with eMacs but with other computers I've used that usually means the motherboard will only recognize that much. So you might find that with two 1-GB chips only 1 of them registers and the other is ignored.
But I'm just guessing. I would go to sites like these...
...and look around. Maybe even write them both an e-mail asking what they think.
I do notice, however, that Crucial.com only lists the 512 chip in their eMac section, so I'd have to say your chances aren't good. Crucial usually knows what they're talking about and if they could sell you a bigger chip I'm sure they'd offer it.
The specification that is listed is the maximum that apple certifies the computer will use, you may be able to get it to recognize more memory, but no gaurantees that it will or won't, I have no experience with trying to put more memory then spec in an emac, but I have done it in some older computers.
Your best bet is stick with specifiaction...
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