New motherboards have a 24-pin power connector. Here is a diagram of both plugs:
You can see that the two are almost identical. The extra 4 pins on the 24-pin connector provide extra connection points for a few voltages. Newer boards and processors need more current, and the extra pins were added to prevent overloading.
Adapter cables are available to go from your 20-pin power supply plug to the 24-pin motherboard connector.
Here's one from TigerDirect, and other vendors have them as well. If you're not making other changes than replacing the motherboard, you shouldn't need more than this adapter.
If you are upgrading your processor along with the motherboard, you should probably replace the power supply too. Older power supplies are often in the 300-watt range, and this is too low for reliable operation with new processors, motherboards and video cards. A new 500 or 600-watt supply is not expensive, and would give you the 24-pin power connection the motherboard needs.
Hope this has helped. Thanks for using Fixya!
I need to add this to my last post. I am upgrading a Power Mac G4, 802 1093A mother board with a 350 MHz processor to a Giga Designs G4, 1.3 GHz processor.
Unfortunately I found out Giga Designs went out of business and I don't know where to find the jumper settings. I don't think you can get into bios on a Mac like you can on a PC.
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