I have multiples of the Optiplex 755 and XPS 420's-
They're great little machines for virtualization.
I currently have them all "maxed" out at 8GB RAM apiece, which is the listed maximum for each.
Often I've found those "maximum" specs to be based on the largest dimm available at the time of publish, and not necessarily a system limitation.
Do either of these machines support 4GB Dimms?
OCZ and Geil both have PC800 4x4GB Kits available, and it would be well worth it for me to upgrade if possible.
Thanks and Best,
Shane