(This reiterates what Fahrvenugen says about IP-address assignments and Network Address Translation but also outlines some further suggestions.)
Routing on your LAN ought to work fine--it's working now, after all. You could even do something cute like putting Apache on your new machine and setting up a little web site on your own intranet. (Or instead of Apache you could use Web/2 or GoServe, both of which let you do server-side scripting with REXX!)
The problem you'd have is routing to the internet. The point of having two interfaces on the gateway machine is to allow it to filter out some of the badness on the internet from the less-protected hosts on your LAN (especially if some are Windoze machines). Sure, setting up the IPGATE ON will let your machine act like a router, but the problem is that only the machine connected directly to the cable modem is assigned a public IP address. The other hosts have only private IP addresses. These addresses do fine for routing on a LAN, but packets with private IP addresses would literally get lost on the global internet. In your setup, all the machines would have to have public addresses in order for the internet-connection sharing to work. This is not a good solution: not only would this negate any possibility of filtering packets, it would mean that your internet provider would charge you a lot more than you're paying for a single IP address.
The solution is a nifty little animal called Network Address Translation (NAT). NAT software, which runs on the gateway machine, modifies the headers of inbound and outbound packets to make it appear that they are all coming from and going to a single IP address.
EComstation 1.1 and later comes with Internet Assistant, which, among other things, does Network Address Translation. I've read a lot of reports of people having problems setting it up, though. While I myself have used all of the packages I mention in this note, Internet Assistant is the only one that's never worked for me. Yet it might work for you--I was trying to apply it to complicated setup. Since it comes with eCS, you might want to give it a try.
InJoy Firewall is a great industrial-strength product that does NAT and has advanced firewall functions. It's hardly free, but it's very capable. I got it way back at version 1.4, when it was cheaper.
What I use nowadays for new installations is a solution that's nice and cheap and flexible: dedicated routers. I don't mean the super-duper high-capacity ones that are part of the internet backbone, but the widely available consumer ones. They might be called cable-modem routers; some of them are also wireless access points. They have the same basic setup as what your gateway machine would need: two ethernet interfaces plus NAT/firewall software. The port that plugs into the cable modem or DSL modem can handle whatever funky login or special protocol that you might need to talk with your provider. You seem to be have the good luck of having to a straight ethernet connection, so this feature is not of so much interest to you. What you might value more is that these boxes also have some firewall function. This protection is above and beyond the protection you get just from using NAT.
These little boxes also incorporate DHCP servers. This lets you add hosts that use dynamically assigned IP addresses if you want. But you don't *have* to use dynamic IP addresses. You can even have a mixed static/dynamic setup.
If you use Internet Assistant or Injoy Firewall or some similar package that runs on your gateway computer, you should stick with the setup you have already outlined: two interfaces, one for the LAN, one for the internet. On the other hand, if you go with the router box, you can take advantage of the fact that it is also a switch, so that both of your computers plug directly into it. On the machine you're setting up, it's the second NIC (LAN1, the Intel one) that gets plugged into the router. The first interface (LAN0) doesn't get used. Leave it in the machine, though, unless you want to go through another round of MPTS and configuration-file tweaking.
You can probably even reuse the same two ethernet cables you're using now. Right now, I'd suppose you have connected the two computers directly using a crossover cable. Cables between a computer and a hub or switch should be straight-through cables, but most switches nowadays can work either way.
I hope this clarifies things a bit.