So I decided to test a couple of 1 GB DIMMs I obtained for a brief period in a Mirror Drive Door G4 tower. It looks like the MDD will recognize the density of 1 GB DIMMs, though any RAM above 2 GB will be ignored.
Essentially, you could have (2) 1 GB DIMMs, (1) 1 GB and (2) 512, or (1) 1 GB and (3) 256 and be alright and things would work just fine.
Unfortunately, nothing above 2 GB appears to be possible.
I had assumed this to be true, but wanted to let you know the results of my unscientific test.