how to DDR2 to DDR3 incrc mac pro

If you are asking how to upgrade from DDR-2 memory to DDR-3 memory, the newer/faster DDR-3 is not compatible with motherboards designed to use DDR-2. To prevent the insertion of a DDR-3 memory card in a DDR-2 slot, the card is keyed differently, with the notch in a different position. This physical restriction keeps the user from damaging a DDR-3 memory card that's intended to operate at 1.5 volts, and not the 1.8 volts supplied for DDR-2 memory.

You Mac is designed for a certain type of memory running at a certain fixed speed. If they could have made it run after with faster chips, they would have shipped it with the faster chips already in place.

Do not waste your money installing faster memory than the specification. It will never run faster, and sometimes does not run at all. In a few cases, it is unreliable -- the worst situation possible.