I'm not sure about all of the potential differences in this case, but my experiences with ASUS networking equipment tell me that the existing unit may be somewhat consumer-grade. You may be smart to leverage the capabilities of the 7100 in that regard.
You'll need to setup one switchport on the 7100 to be in a separate VLAN, say VLAN 100. VLAN 100 will be your public connection and the modified switchport will connect to the ISP equipment/modem. VLAN 100 will need an IP interface where you'll assign public IP(s) or configure to receive DHCP from the ISP (for the public connection, not for inside devices). The firewall must be configured so that VLAN 100 is in the Public (or similar "outside") security zone and the firewall must be configured as usual. Here's a guide about configuring VLANs and VLAN IP interfaces, with examples: The difference between VLANs and VLAN interfaces. Here's a great starting point for firewall configuration: Configuring Stateful Firewall in AOS, and an important explanation about how the firewall menu behaves in the web GUI: Understanding the Firewall Menu in the AOS Web Interface.
Regarding the NAS, I think patching it to a gigabit switchport is a good move. You're still limited to 100Mbps for any one user, but it will help when multiple users are communicating with the NAS and mitigate bottleneck in that regard. If you have any file servers or other needs for high-throughput applications, uplinking from the 7100 gigabit port to a gigabit switch where the NAS and servers connect is a good idea. The NetVanta 1531 is a superb value for small offices!