Hooking up 2 computers to DSL

Just get a router.

Grumpy

A router can be attached to your current modem and essentially give you 4 or more ports of Ethernet to run to your systems. If you find it troublesome to run Ethernet, you could invest in a wireless router and either wireless NICs or wireless 'ports' that plug into USB on the systems. If you do choose wireless, remember to use WPA2 for encrypting the signals.

Side Note: Your ISP may require you to pay more monthly to have more than one system connected.

It would make sense to get a wireless router, that way you're not running wires all over the place, and a wireless card for her machine. You'll hardwire you computer into the router. If the Gatreway isn't running XP, it will make your life significantly easier to get an OEM version. Ended up doing this on an old 865PE running 2000 that wouldn't recognize that the wireless card was plugged in. I've set up old computers for my kids and grandkids and would suggest in addition to XP you put in 1GB of RAM and a new 7200 rpm hdd. This should keep it running smoothly until you need an excuse to build yourself a new machine and pass the current one down. At least my wife has not given me a hard time about upgrading when I'm doing for the grandkids.   The main reason you want it running smoothly is that you are the tech support and kids don't have a lot of patience when something isn't working and have a way of letting you know.