VLC is not well optimized for AltiVec. Some people have reported decent performance with Movist or CorePlayer, but I just use QuickTime 7 and Perian playing back H.264 on my Luxo G4. It works well enough.

I'm not sure, otoh, which other codecs can be automagically shifted off the CPU onto the GPU for decompression, and what the OS and software requirements are for that. It appears your GPU is not one of those supported by CoreImage. So it may be that DVD/vob/MPEG2/MPEG1 files are the only ones which will run entirely on the GPU.

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On paper, there is no GPU codec acceleration on Power Macs, just GPU scaling of the output. However, Apple does have some private interfaces that accelerate some codecs, H.264 believed to be one; it is probably the most accelerated of the codecs, but you will only get this going through QuickTime. 3rd party apps implementing their own decoders, including ffmpeg, do not have any way of running on the GPU in PPC OS X.