1.0GHz and 1.25GHz "USB2" eMacs had the capacitor problem -- I believe it'll be a version with ATi graphics.

Here's Apple's info page about it: http://www.apple.com/support/exchange_repair/emac.html

If it's in that serial range, don't plug it in anymore, get it recapped or take it to an Apple store to see if they'll repair it for you. (Apparently some of them are still doing the repair, but your mileage may vary.)