It's dead easy.
Best thing you can do is to read the manual, which you can find on you Mac Pros hard disc. The manual is called Mac Pro users guide.
Just make sure that you fit the 4870 in the first bay and the GT120 in the second or third.
As easy as it is, you need to be careful about not to jostle your MP around too much. I unseated one of my RAM sticks during the installation, which made for some anxious monuments (MP would not boot). I suggest removing your memory risers first. This will give you more room to work, and you can reseat the RAM sticks before putting the risers back in just to make sure there are no nasty surprises.
As easy as it is, you need to be careful about not to jostle your MP around too much. I unseated one of my RAM sticks during the installation, which made for some anxious monuments (MP would not boot). I suggest removing your memory risers first. This will give you more room to work, and you can reseat the RAM sticks before putting the risers back in just to make sure there are no nasty surprises.