I remember at one point reading what OS X version officially depreciated these machines; it was quite an old version (The retail version of Leopard?), so while it might be "fun" to try to put a more modern version on it I suspect you will have to treat it the same as if it were an equally old generic PC and, indeed, "hackintosh" it. (I can't imagine the rest of the hardware being good for anything past Snow Leopard, or possibly Lion, anyway.)

In any case I would personally stay *far* away from upgrading the CPU; while I sort of snicker at "prototype collecting" I do imagine the surviving DTK's are going to be at least somewhat collectable and so far as I'm concerned it's *way* too easy to screw up LGA775 sockets to risk it. Is the board's form factor standard mini-ATX or is it "super proprietary"? If you *really* must try to put Yosemite on it I'd say *carefully* pull the original board and keep it nice and safe.