Mad Man with a box
No, they can't.All x48 boards can take up to 64gb of ram.
You will not be able to boot from a 4GB DDR3 Single Side stick or 8GB Dual Sided stick, regardless of what you do or what MB* you have (*non-server grade).
Also, there are no DDR2 8GB/16GB memory modules to buy for consumer grade boards.
Without ECC/Buffered memory support on MB/chipset side you can't make them work.