I believe primarily it's the Memory controller on the A64, as I've seen people having all kinds of problems with 4 x 512MB on different Manufacturers MB (ASUS, DFI, MSI, etc.). I'm not sure if single/double sided makes a difference.
Even dropping Memory Divider and Memory timings haven't solved the problems for some.
I know if/when I decide to go to 2GB, I'm just going to get 2 x 1GB and avoid all the headaches 😉