Such a large question isn't going to get a step by step answer, however I think covering the general way to do it is easy to do. And if you have any q uestions about any specific portions, that can be answered pretty quick. The short answer is, you would share it the same way you share any other connection (whether it be cable or dsl.) You're going to have to network your computers, but you knew that.
Get one nic for each computer, a hub, and two cat5 patch cables. (You could also forget the hub and go with one crossover cable instead, but if you ever wanted to add more computers you'd end up buying a hub anyway, plus if you decide to use a seperate box for routing (good idea) you'd need a hub.) Give the computers static IP's on the network (like 192.168.1.1 and 192.168.1.2) and you'll have a network.
Now, with the two computer model your computer dials, and you run maquerading (I wouldn't be surprised if RH8 had some sort of wizard to set that up for you, and there are many firewall scripts you can find that will work with very little tweaks.) Your wife's system you'll put yours in as the gateway, and the connection will be shared.
An alternative method which I like better is to have a dedicated routing box. Any old thing, a 486 dx2 66 is routing a dsl connection for my mom. You can either put a hard drive in it and setup a linux install to do the routing, or use one of the floppy routing distributions. Freesco would work simple, and freesco is quite easy to configure. With using a seperate box for the routing, it would have an IP on your network (you need a hub for this, the crossover cable's only good when using two computers.) and both your and your wife's comptuers would set your default gateway to that.
Well, that should get you started anyway, best of luck mon ami!
Edit: Oh yeah, also some of the linksys style products are capable of sharing a modem as well. You won't have as much flexibility as a linux machine doing the routing, but it would be the easiest way to go.