10.6 and 10.5 (server version) will happily (well, for testing, at least) run in a Virtualbox VM. The host OS have to be OS X server too but with the 99$/year you got also server license for free. Using server instead of desktop is not a significant difference if you don't start the server daemons... somewhat remembers me of NT workstation and NT server (many years ago). :)

It's quite easy to set it up to boot of an external drive; I've done that and had 10.4/10.5/10.6 all bootable off of one external drive (split into three partitions).

Create multiple partitions on the HD - 10.4, 10.5, etc.