PPC and Intel are not at all interchangeable. The closest you could manage would be gut the case and make a hackintosh, by finding an intel logic board that would fit inside (may not even be possible).
My advice is to keep using the old software as long as you can. Look for backward compatible versions of newer software.
You should look for a good deal on an Intel Mac you can max out the ram on (should be cheaper than getting a new box). Also upgrading CPUs are not simple DIY tasks, lots of dismantling of the machine involved, and limits as to what OS runs on what board/processor.
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