Tricky ones. I have never tried a 2GB stick in a G4, but I have heard of oversized sticks working in Desktop G4s so there is a chance it will work. I would think the last of line 1.67GHz DL-SD model would be more likely to work since it uses the newer 5300 RAM, but I honestly don't know. You might find the answer on XLR8yourmac.com. Same applies to hard drive sizes. They'll be in the drive upgrade database. I suspect Alu PBs are all fine with bigger drives.

PBs should get no speed benefits from matched pairs of RAM. That one I CAN answer.