I've just installed my new memory units on the Mac and this was actually quite easy.
My Pro came with two 1 GB memory units and since I'll be using some quite memory intensive applications and also use a virtual PC I wanted more memory. So I ordered a 4 GB memory pack, which comes as two 2 GB memory units.
The first step was to remove the bottom cover for accessing the contents of the computer. My first attempt didn't work though. I was using a very small screw driver, but apparently this was to large. I managed to release three screws but not the others. I was perhaps a bit to careful but a the same time I really didn't want to break the screws and thereby not being able to remove them at all. A few days went by and finally I managed to get hold of a screw driver with the right size.
After removing the screws and then the cover the memory units were placed almost in the middle (sorry for not providing images of this). I saw that these where fastened with screws so I started removing these too. I later realized that this was totally unnecessary since all you have to do is expand the constraints to remove the top memory unit. Under this the second unit is placed and you remove this by expanding the constraints again.
After this you just click the new units into place, put the cover on again and you are all set. Not that hard at all.
After restarting the computer I noticed an improvement in speed, especially while using the virtual PC.