This page helped me immensely in figuring out what to change the jumper settings to in my Beige G3. I went for the stock PCI and system bus speeds to stay on the safe side, as well as the rated clock speed for my CPU. You do need to change the jumpers otherwise your new CPU will run at 266 MHz - you'll need to use a pin or similar to extract the metal bits from the jumper block in order to change them around
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Main system: custom C2D 2.66GHz tower/4GB RAM/2600XT 512 MB/500GB HD/XP Pro x86+Vista Business x64
Main Mac: Beige G3 minitower (Sonnet G4 500MHz/640 MB DVD/USB/Radeon 32 MB/OS 9.2.2+10.4.11)