How much is your time worth to you? The reason I ask is because what you want to do will take a
really long time on a G4 800.
The other option is to get a standalone dvd player that also plays divx/xvid files. I have a philips dvd player that i got for 39 dollars on black friday last year. I burn about 5 movies to a DVD/RW (or a single movie to a CD/RW) in its native AVI format, then play that disk on my philips player. Once I've watched the movies, I erase the disk and put 5 more on.
Takes about 30 minutes to have 5 movies ready, versus several hours to have one movie ready.
Here's an example:
http://www.bestbuy.com/site/olspage.jsp?skuId=8216414&st=divx&lp=1&type=product& cp=1&id=1166840037410
If the link doesn't work, go to bestbuy.com and search for the sku: 8216414