Kenneth Griffin,
There is no supported method of making that upgrade. Even the "maybe" solution offered in this thread would not work, and if somehow managed to make it work definitely not be supported. In addition it would be more expensive than a new machine.
There is a solid market for used Macs, especially high end PPC machines like yours because some businesses and others have not made the transition to Intel yet. For example some are still using business or education applications that require Classic. I am not making a guarantee or suggesting you would be able to sell your current machine, but rather suggesting it as something to look into.
If you are primarily looking to make the change for games and not other applications, in addition to the Mac Pro, you may want to consider an iMac. All of the current Intel based Macs are multicore, even the Mac Mini. The Mac Pro's offer up to an 8-core configuration.
Hope this helps,
Nathan C.