I had a 600MHz G3 white, 12-inch dual-USB iBook that came with Jaguar and 256MB of RAM. I eventually upgraded the RAM to the max 640MB and put in a 120GB hard drive and installed Tiger, and it actually ran a bit faster than it had on Jaguar. I would still be using it this very minute if the logic board hadn't died (a common issue on this particular model) on Saturday. If your PowerMac is comparable to my iBook, I can say with confidence that you will be better off with Tiger than Jaguar. Not only will it be a bit faster, but you will gain so, so much from a usability standpoint.