I am having problems with the memory on my Apollo board, only half of it seems to be available.

It is most likely that you have EDO ram fitted on the Apollo accelerator, which it does not like. If possible try to find one of the old FPM (Fast Page Mode) SIMM's. These are becoming rather difficult to find in most areas of the world and retail at a slightly higher price than EDO. It may not fix the problem, but there's more likelyhood that FPM will be more compatible than EDO.