SBS 03 Sonicwall 2040 Help

[Amitymm]

Hey all I am new to the forum, want to say thanks in advance for all the help. Ok so my setup is pretty basic at my business I have a sbs server 03 it is acting as my domain controller, dhcp server and running Microsofts isa firewall software also has its own internal dns. I just purchased a Sonicwall 2040 Firewall in hopes for better security. Now my server also has 2 nics one wan one lan. Basically I have no knowledge about hardware firewalls the setup seems easy enough, but somethings that I dont want is I still want my server to be the dhcp and assign the ip address. Now I know the firewall has to be in front of the server and it needs a ip address. So basically how do I go about this setup I want everything to work like it has with this firewall in place. I am just wondering what ip do I set the firewall to, and does it take the server's ip or one of the range of ones my isp supplied to me? If I can get any suggestions on how to set this up that would be great thanks again.