Is Windows 2003 firewall safe?

I want to setup Windows 2003 + Exchange 2003 server with to network
adapters: one for the intranet, and the other for the internet.

Is it safe to use Windows 2003 built-it firewall on the external adapter to
block everything but mail and dns ports (53, 25, 110, etc)? If not, can I
install ISA 2004 firewall to protect it (i.e is Exchange 2003 compatible
with ISA in the same machine?)

Ideally it would be better to have an external firewall but I can't afford
it ($$$).