New logic board...I would start with a fresh system hard drive. Carbon Copy Clone your current HD to a FW / external drive and wipe it clean...then do all fresh installs of your apps.
You have bigger stones that I, my friend. My G5 Quad died last week and I refuse to pony up $1000 for a logic board. I am selling it for parts. I took it as a sign that I was meant to move on to Intel.
When life gives you dilemmas...make dilemmanade.