Nothing you can do, sorry. This isn't an issue involving a lack of video RAM, it's a lack of processing power. Sorry to say, but the world has passed the G4 by, and it's really just not able to keep up anymore. Videos are often higher quality these days, which means more data to process, and there are often embedded support scripts, etc. Flash was never all that great on the PPC platform, and with the shift to Intel CPUs 3-4 years ago, there's less and less incentive to really bother improving it.

Since that thing is likely well out of warranty, you can investigate options for overclocking the CPU and GPU, but be sure you understand the risks. I'm not really sure you can really overclock them sufficiently to make much of a difference though.