My brother, is using a Compaq Prosignia Desktop 320, with a 64 MB
SDRAM PC100 - 100MHz 168 pin DIMM memory module. The mainboard's
(Intel B1440ZX) memory maximum is 256 MB. We recently ordered an
additonal 128 MB RAM module from Crucial.com.

MAke sure that the new RAM is not DDR.

The mainboard has two RAM slots (1 and 2):

Slot 1 = 64 MB

Slot 2 = 128 MB

I have never found it to matter where it's placed(and I've done a lot of RAM
switching). Yes, PC100 RAM too.