Acer memory upgrade issue

Greetings. I have this clients four year old Acer Veriton X480G that I can't seem to get the memory upgraded on. It has four available slots with 2GB of GU512303EP0202 memory. When i get the application it calls for all the memory upgrade modules make it have a solid long beep in the bios.

I read in one topic that it was because the processor needed to be dual core - So I took out the E6300 and installed an E8500.

Then another topic told me that it was because it was a buffered vs non-buffered memory module and that isn't so because the module that is in it is already a non-buffered memory module.

Anyone else ever run across this issue? I have four of these desktops that I need to upgrade and would appreciate the advise.