You cannot host multiple Commerce Server 2007 Web Applications on a single server of any kind using host headers. Because the web services that support commerce server require SSL for client connections, each Commerce Web App must be configured to support SSL.
Host headers are not supported under SSL (see http://support.microsoft.com/kb/187504).
Your only choice then is to use a technique other that host header to support multiple Web sites on your single server - either multiple IPs or different port numbers. Remember if you use different port numbers you need them for regular HTTP traffic and secure traffic (e.g. 80/443, 8000/8443, 9000/9433, etc.)
Note that if you are using the CS2007 Starter Site that code changes are needed to support a non-starndard ports (something other than 80). See this blog entry for details.