ISP to a firewall/router solution.

I have plans of doing business for a start up by utilizing the pricing of coax cable in my area.  I am looking at getting a 50 mb connection with one static ip address at a very good price.  However I have a question about how I would configure my router/firewall.  I have a Netgear Prosafe FVS318G that i would like to utilize.  So when speaking with the cable rep he is telling me they would supply a piece of equipment (i'm assuming a cable modem) that would be set up in "pass through mode" (are his words) and we would get a Ethernet hand off from them.  So I would take that hand off and plug it into my Prosafe FVS318G.  When configuring this router it asks for when using a static ip.  1. IP address (which would be my IP address that the cable company would give me)  2. subnet mask (which the cable company should supply. 3 The default gateway.  Here's where i am a bit confused.  What would I be listing as the default gateway?  As their equipment is in pass through mode and my netgear prosafe would be the gateway for my network?  I only am paying for one ip so would i put that ip in both IP address and default gateway or would i leave the default gateway blank or can I not use this piece of equipment as a gateway router?  I feel stupid for asking this question and feel i should know the answer to this but for whatever reason I'm drawing a blank.

Any help experts?