remember, imovie and idvd are resorce hogs when encoding. i use ilife05 on my sp466g4, it slow, but gets the job done. i would get the tower for one main reason, its upgradeable. and the 1st thing i'd do is max the ram out to its 1.5gig, the g4 mini is only 1gig. that is going to be the best thing you can do to speed up things. plus the dual 800 has l3 cache, which really helps alot to do imovie and idvd. i have seen bench marks that put a dual 800 w/l3 cache as fast as a dual1.25 w/out when it comes to qt encoding. also if you want, you can upgrade the cpu to dual 1.8. and because it has pci slots, you can buy interface upgrade cars, like a sata150 and matching hard drive, which helps big time. the g4 mini's hd interface isn't any faster then 100, and the drive doesn't spin faster then 5400rpm. sata drives interface is 150, and most spin at 7200rpm or faster. plus they can hold alot more. on the mini you will run out of room fast, but you can always add more to the tower.
so to make a long story short, the dual 800 is the better buy, and it will be useful much longer then the mini because it can be upgraded.