i am 5' 8" and my measured standover is 30.5". according to bikesdirect.com and their motobecane's, the 16" frame is for riders up to 5' 9" and the 18" frame is for riders up to 6'. they only have 18" frames in stock. will the 18" frame be too big for me?

probably..wait for the 08's to come out....and get the bike that fits you....the other thing you can do is compare geometry to the bike you're riding now...i'll send you measurements if my bike setup if you want, and you can compare them to your current bike, and that'll help GIVE YOU AN IDEA of what an 18" frame will be like....