A week should be plenty. Having spent a lot of time in the area over the years on the motorcycle, some of the must sees are Mt. Rushmore, Crazy Horse, and Custer State Park (particularly the Wildlife Loop). The roads are great whether on a bike or in a Jeep... Needles Highway and Iron Mt. Road are awesome! If you have time, Devil's Tower in Wyoming isn't all that far away and worth the drive.
This past fall for the first time I spent a couple days wheeling in the Black Hills... Hal John's is an excellent trail if your looking for something difficult, they also have what they refer to as the "cereal trails" made up of Got Milk?, Fruity Pebbles, and some others. I was hard pressed to find guide books, etc., but the clubs up there were more than helpful and willing to get you pointed in the right direction.
You'll have a blast! :biggrinbounce: Remember to post up some pictures.