i installed 10.3.8 yesterday, and it put me thru hell. There seems to be alot of problems people are experiencing.
I had a joystick controller driver on my system that didn't agree with the new update, and for some reason it was hogging all the processor power. My G5 was acting like a very sick g3. Fortunatly there was someone else in my position before me and they figured out the issue and reported it on the apple discussion forums for me to read and fix the porblem without bashing my head thru my LCD display.
MacFixIt.com is a website that is cataloging all the problems that people are having with 10.3.8. It helped me greatly. Have a read thru it and see if you have a similar system or issue.