I primarily use a quad G5 during the winter months and a 12" powerbook over the summer months. When a website is a little too javascript heavy for either system, I VNC into a generic headless xeon desktop box I have running Linux in my basement. VNC enables me to extend the life of my ppc macs with a bit of cheating of course.
One of the best things I've figured out is that you can set up screensharing on your PPC Mac and set up Chrome Remote Desktop on your modern Mac. Then you can use your PPC Mac from anywhere you have an internet connection!