G4 to slow

Any application I tried, it's very slow, not even able to look at a simple video, got the sound but the image flow like to freeze end come in sequence, asking for better solution.
I got a G4 with 400Mhz and 1.5ghz ram, which processor I can upgrade and I'm with the OS_X 10.3.9, can I upgrade to an higher OS or do I better get lower OS, if so which one.

A faster processor would obviously help, however upgrading to Tiger would probably slow it down even more. I hate to say it, but that's a really old computer. Start getting used to it