Multiple Wireless Standards a/b/g/n, must I use the slowest?

I have six laptops that are a mix of xp, Vista, and Windows 7.  Wireless cards are a mix as well with  b/g in some and  a/g/n in the newer boxes.  My old b router worked with all boxes, presumably at b speeds.  I want to upgrade the router.  Must I upgrade ALL client machines to an n card if I wish to take advantage of the newer n router speeds?  If I don't do this, will the new n router simply work in the slower b mode for all clients, even the ones with a/g/n?  Does a dual band router get around this issue?