If there was only 1 memory stick in there to begin with, it can't be a dual-channel error.
Found this:
http://www.laptopmag.com/advice/how-...000he-ram.aspx
It seems you need to enter BIOS (hitting F2 a lot at start-up) and see if it reports the new RAM size, then save it manually as the system (for some reason???) doesn't save the changes on it's own.