How well does streaming work for high definition files under plex?
It depends on the bitrate. For example, the Apple 720 HD files are encoded around 4 to 4.5 mbps and stream just fine. However, ff and rew are pretty slow. The few transcoded Blu-Ray movies I have (1080p down to 720p) are encoded at 6 mbps and drop frames every now and again to my Apple TV, but not to my Mini. The principal difference here is my ATV is on WiFi-n and my Mini is by ethernet (100 mbps). My Blu-Ray movies stream fine through Plex using ethernet.
I want to do this on my my 1.25GHz G4 PPC Mac mini with 1 GB Ram and 100 GB HD Can anyone help me?
You'll likely need to get a different computer. The G4 isn't a great processor for doing 480 video playback, let alone HD content, and I don't think it even came with Front Row or a remote (i.e., no IR), did it? The Intel dual core Minis are probably the lowest for playback of HD content, while a single core can handle standard def (e.g., DVD rips).