You're dreaming if you think you can decode 720p broadcasts with a G4. According to Elgato, the system requirements for it's EyeTV hybrid TV tuner are:

System Requirements

* Macintosh® computer with a PowerPC® G4,G5 or Intel Core® processor

* 512 MB of physical RAM

* a built-in USB 2.0 port

* Mac OS X v10.4.11 or later

* Internet connection required to download Program Guide data

Note: 720p or 1080i HD features require an Intel Core 2 Duo processor. Analog TV recorded and processed by a software encoder; quality depends on processor performance.