Two words: update everything
Run at least 8.6, ransack Apple Software Udates, nit-pick Versiontracker.
If the power seems to be a problem, unplug the monitor from the back of the computer if it's attached there. I actually did see someone's power supply die with a 17 or 20 inch _Apple_ monitor plugged into it. It was an 8500 or 9500 too.
I'm running an 8500 with a PowerLogix 604e overclocked, Rage Orion, MacAlly USB card, and ProMax IDE card (very trick). What type of USB card is he using? What kind of devices are hooked up to it? Perhaps a powered USB hub will take some load off of the PCI card.