DSL router
IF you have a DSL router, you're set. It will be the gateway to the Internet for you local network.
Get a wireless hub or switch and using the "uplink" port on the hub/switch connect it to your router with a short piece of Cat 5 networking cable. Your PC's will naturally need a wireless transmitter card installed in each to communicate with the wireless hub/switch.
I don't think that a DSL modem by itself can share PC's. A DSL router/modem certainly will allow multiple PC's to share one connection.
Try this link to see the 4 port Linksys DSL router/modem
http://www.linksys.com/products/product.asp?grid=23&prid=20
One other thought, Win98/ME and I 'spose XP can be configured to "share" modem connections. (So can Linux, but heck, it does everything better than Windows. What else is new?)

Therefore, it may be possible to use one networked PC to act as the "gateway" for the other networked PC's through the wireless local net.
I haven't tried this 'cause on my local network I dial up to the Internet through a 56K modem/router. It does the sharing. Don't have to have a particular PC up and running to access the 'net.
However, a DSL router/modem can act as a firewall, which is a good thing for 24/7 connections to the Internet.
Tim
Late bulletin: A little more research on the Linksys site yielded up this little beauty (it is wireless!) Oughtta be just the ticket!
http://www.linksys.com/products/product.asp?prid=173&grid=22