Go here for parts and a diagram: http://www.welovemacs.com/emsepa.html
Also hook it up to an external monitor to try to isolate the problem. if it works then it's the CRT.
According to which eMac you have, the following applies:
You have to have an Intel processor to run Snow Leopard.
Mac OS X v10.5 Leopard requires:
- A Mac computer with an Intel, PowerPC G5, or Power PC G4 (867 MHz or faster) processor
- A DVD drive for installation
- 9 GB of available disk space or more
- Some features require a compatible Internet service provider, fees may apply.
To use Mac OS X v10.4, your Macintosh needs:
- A PowerPC G3, G4, or G5 processor
- DVD drive (DVD-ROM), Combo (CD-RW/DVD-ROM) or SuperDrive (DVD-R) for installation
- At least 3 GB of free disk space; 4 GB if you install the XCode 2 Developer Tools