May have narrowed down the problem a bit. I'm using single-sided 1G Memory cards. When I insert 2 DIMMs I only see 1G, when I insert 3 DIMMs it shows 1.5GB of memory. Seems like it is only reading 512KB per DIMM.
Interestingly the BIOS says they are running at DRAM Frequency 200Mhz which I understand is really 400Mhz. The manual says that with 3 DIMMS the speed drops to 333Mhz.
Still don't know why it is only reading 512MB per DIMM.