Setup Modem/Router and AirPort Extreme running DHCP, NAT

I have a basic modem/router from my ISP which didn't always work very well, so upon suggestion instead of upgrading to a new one I purchased an airport express to use with the modem, since we mostly use apple devices at home.

The idea is to use the old modem/router strictly as a modem and use AirPort Express for everything else (routing & access point).

However I need some technical info to set this up, since I not very familiar with networking.

This is what I understand: In my old modem/router, I need to turn off wireless (it will be connected to the airport via Ethernet cable) and DHCP and NAT (if I can figure this out) as well as the firewall. After that, I should connect the airport to create a new wifi network and have it run DHCP & NAT (which will also provide firewall services)

Does this sound right?

Also, the Airport product page mentions that it can also run/provide the following: PPPoE, VPN Passthrough (IPSec, PPTP, and L2TP), DNS Proxy, SNMP, IPv6. Sorry if it sounds ignorant, but do I need to bother with any of this? Specifically PPPoE, which sounds like a modem function.

Again, since we're talking about a crappy modem here, the idea is to let the Airport do most of the work, and leave the least up to it.

Thanks in advance.