I would check EXACTLY what hard drive is currently in the computer. You can check this in system information under ata. Get the serial number of the HD, then cross check that serial # with the manufacturers size specs (for that particular product) on their website. Then, check and see if the Maxtor has the same dimentions, etc...
The only thing left to ask is... Do you have th balls to take that apart?
It can be done... i read about some dude who overclocked his 800mhz to 1.33ghz (and stable!) I don't remember the site... but he also did a hard drive upgrade too. Just run a google search on 'overclock emac', or whatever. Many of the sites at least tell you how to open the case.
Yes, the HDD can be replaced with any standard IDE drive. As for if you can do it yourself, well, that depends on how brave you are. The hard drive is probably the most difficult component to replace in the eMac, as the entire logic board needs to be removed to get to it. For someone who has never opened up a computer before, I would *not* recommend you do this.
However, if you're still willing to try, download the eMac service manual and take a look at the steps required to remove the HDD.