You should be able to swap the cards, as all they each require is a free AGP slot (according to ATI). Remember, the Sawtooth only has a 2x AGP slot and the MDD has a 4x AGP slot. I would think the 4x AGP card wouldn't be compatible, but ATI doesn't say that. All they specify on their website is that the Radeon 9000 requires any AGP Powermac G4. So they should both be compatible, but your Radeon 9000 will probably seem even slower in your Sawtooth (since it's now on a 2x AGP bus), while your 9800 will feel even faster in the MDD (4x AGP). As far as upgrading memory, the memory is onboard, which means it is not upgradable. If you wish to have more memory, you will need to purchase a new card. The Radeon 9800 PRO is the fastest card that the Sawtooth can hold, so if you were going to upgrade a card, I would suggest leaving the Radeon 9800 in the Sawtooth and getting a 4x AGP card with 256 MB of memory and putting it in the MDD. That would probably be the most cost effective, since a 4x AGP card will probably be similar in price to the 9800 pro.
As far as taking all of the upgrades out of the sawtooth-that should be fine but remember your MDD has a faster system bus than the 100 mhz Sawtooth bus. Depending on the MDD model, it could have a 133 mhz or 167 mhz bus speed, so taking PC100 RAM out of your Sawtooth won't work in the MDD and you will have to buy new RAM. Other than that, the PCI cards and hard drives/optical drive should all be transferrable.
Hope that helps!