When you go in and assign the port and IP to each web site, you also have the option to enter the host header. Set that there and you should be done.
One note, if you have, say, two web sites on the same ip/port and one has host headers and one does not, the one that does not will get used if a) someone has a real old browser that doesn't support host headers or b) navigates by IP. Often people will put a default website here with links to the domains that normally use host headers.
Let me know if you can't find it, I'm running a handfull of sites at home with them so I'm pretty familiar with it.
Bill