Do you get a boot chime after a PRAM reset?
Restart while holding Option-Command-P and R. You may not hear the boot chime, but continue holding the same 4 keys for about 60 seconds. If you DO hear the boot chime, hold the keys until you hear the boot chime two more times (or about a minute, whichever is longer)
On a side note - I always recommend checking for swelling capacitors, particularly on the USB 2.0 models. The swelling capacitors will cause a variety of issues over time. Remove the main cover, and the internal shield, which will allow you to see all the capacitors. Swelling (or even leaking brown goo from the caps) will be easy to see. There have been repair kits that will get the correct capacitors. Don't know if they are still available as a stock kit, it's been several years since I last looked for those.
(found a parts kit, along with a simple picture that shows what swelling looks like!
http://www.amazon.com/eMac-motherboard-Capacitors-repair-Kit/dp/B00FJRX35S )
Also, check or just replace the 1/2AA battery. They WILL go flat if the eMac has been in storage (unplugged)