I have a G3 Pismo Powerbook, with an 128 meg, 100 MHz memory module, to which I added a 256 meg, 133 MHz module of another brand, and, to my atonishment, everything has been working flawlessly for 6 months now.
So, I suppose that your 512 Mb + 1 Gb won't give you any problem, that is IF your G4...