Getting the amount of memory installed in a machine with WMI is a bit confusing particuarly if you only read the docs partially. I was initially using Win32_ComputerSystem::TotalPhysicalMemory, but the documentation warns "Be aware that, under some circumstances, this property may not return an accurate value for the physical memory. For example, it is not accurate if the BIOS is using some of the physical memory."
The suggested alternative is Win32_PhysicalMemory::Capacity. This was an easy switch in my script, but, I was getting numbers I knew were wrong for the machines I was querying. The part I didn't read was that each instance of Win32_PhysicalMemory represents a single stick of RAM, so, you need to loop through them all and take the sum to get the RAM installed. This snippet will get you the total memory in megabytes:
Set colItems = wmiSvc.ExecQuery("SELECT * FROM Win32_PhysicalMemory", "WQL", wbemFlagReturnImmediately + wbemFlagForwardOnly)
totalMemory = 0
For Each item In colItems
totalMemory = totalMemory + CLng(item.Capacity) / (1024^2)