Well, with the cost of Dreamcast's right now, its actually probably cheaper to get a whole new one, than get the motor, which I think you have to send it back to Sega to get a new one(your warranty will cover part of it, but the cost is something like $70 plus S&H.)
Best Buy is selling new DC's for $49.95. BTW, if you have a Dreamcast, you *must* get Shenmue2. I just got it yesterday from England. It works with a boot disk. That game is GREAT. Supposedly the XBOX version isn't going to be as hot. --Deji