Rico, you're making this more complicated than it needs to be.
Bottom line - the software restore discs that come with each machine are tied to that specific model revision. That is why your boss's new discs wont work on the old machine. Virtually all hardware companies do this - Dell, HP, Acer, etc.
Retail copies of MacOS X will install on any Mac, provided it meets minimum requirements (which yours does).
If you don't want to pay for a retail copy, and your boss can't find the discs that originally came with the machine, then call Apple and see if you can get another copy. They might make you pay a nominal charge, but it will be less than the retail price, and will likely include whatever version of iLife the Mac originally came with (presuming you want it). Retail versions of MacOS X do NOT include iLife.
You could install your HD in your boss's new machine and install the software that way, returning the drive to your machine when completed; it'll work, but that would violate the Apple end-user license. And you'd need to repeat the process should you ever need to re-install the OS in the future.