Linksys Wireless Router :: Limited IP Addresses On E4200

setup the network at his church, they currently have a Cable modem, which hands out 3 external IP addresses, I have that plugged into a Linksys E4200 wireless router, and from the router a line into a netgear 48port switch which hard wires into the offices and a few more E4200s around the church. The linksys is currently handing out IP addresses to all the equipment on the network, the trouble is it only has a limited amount of IP addresses,(50ish wired?) and i am worried that it will become overloaded.  They are also trying to go gigabit as much as possible, as they pass a lot of files around internally.

1. Am I using the right piece of equipment to hand out the IP addresses? The modem will only put out 3, so do I need something instead of the E4200 to do this? If so any recommendations?

2. Do they make a switch that will plug into the modem that is gigabit that will handle internal IPs? I am happy to trash the netgear switch if there is something better to replace it.

3. How can i utilize all 3 external IP addresses? (Well 2 really, I have their VoIP phone system plugged into 1)
 
I am a low voltage contractor by trade and A/V is usually my department, I would like to get into networking, but on a scale of 1 - 10, I am only around a 4 in the knowledge dept.