I'm with sjau - Debian for servers. I would have suggested eBox to meet most all his SBS needs, but they are moving to Ubuntu.... Not my choice for servers.
SME server offers much of the same functionality in an easy to install and configure package. Based on Centos, it is pretty rock solid.
Having moved several NT and W2K domains to linux, the real challenge is migrating the users with the appropriate permissions. The idealx scripts were decent for a Debian from scratch install, but with the pre-fabs like eBox and SME server, installing the scripts can break your install.
for information on migrating Windows domains.