As I get older and more years of AutoCAD under my belt I find myself not remembering all the command and system variables. I know you cannot memorize them all. I even wrote a command and system variable map years back and now Lee Ambrosius updated and improved it. With AutoCAD 2006 there is a nice helpful way to jar the memory called AutoComplete. You must at least guess what the command or system variable starts with.
Start typing at the command line the beginning for example I was looking for the system variable that controls the mouse wheel zoom speed. I typed ZOOM and then hit the TAB key and get the options and found ZOOMFACTOR. Funny I forgot as I had that system variable default changed from 40 to 60 back in AutoCAD 2004. Wade through the dimension system variables and commands with DIM or when you are really forgetful just enter D and go through everything that starts with the letter D.