Disney Vacation Club members have rolling "use years." It is a bit confusing, but all you need to know about what this means is that DVC rooms are continuously being offered to regular, non-DVC, guests. There is no specific time of year you need to call. DVC rooms may be available to regular guests up to 11 months in advance. For example, I just tried to make a test booking at the Saratoga Springs DVC resort; I was able to make a reservation into November 2010.
DVC villa reservations can be made online. To do this, go to the Booking section of the WDW website, found
HERE. Toward the middle of the page, you will see a button called "Advanced Search." Click on this and you will be given the option of reserving a villa room in one of the DVC resorts.
I have stayed in villa rooms at the Wilderness Lodge, Beach Club, Saratoga Springs, and Animal Kingdom Lodge. I find that the extra space makes my family vacations much more pleasant.
Good luck with your planning.