Power Mac G51.8 DP (PCI) ram not being seen!

It seems like two of the ram slot in my computer won't work. There are 4 slots in this model and I currently have the computer working with a 512MB chip in the very top slot, and another 512MB chip in the very bottom slot. The two middle (Inner) slots are having problems. If the ram is put into the inner slots, the computer wont start up at all. Sometimes even with the current way I have it set up, the computer will not fully start up after being turned off. It will stay at a gray screen with the spinning wheel cursor. The computer randomly won't make the chime noise upon start-up either. I did a little research about this problem, and I don't know if this may have anything to do with the PRAM battery needing a replacement. This computer about 6 years old and I never replaced the PRAM battery. Any input would be helpful at this point. Is there any way for me to get this computer up and running with the four the 4 chips I have ready for it.

Thanks.