win 2k security

in our organization we have one win 2k server and 30 workstations. we use internal IP and we are conected to the net useing DSL and a a modem that acts as NAT. recently I insatalled exchange 2000 and we opened the e-mail ports on the modem to my server so we can send and recive from the interent. now i was wondering if i need to install ISA server on my server for security reasons or i’m fine since my server is behind the modem’s NAT. I’m asking this b/c i’ve heard of many problems that ISA canhave on a PDC.
please let me know of your experienced with ISA 2k

thank you