Well, you've got a lot of options available to you.
Firstly you could buy an all-in-one modem/wireless router that also has network ports. Something like the Linksys WAG54G or Dlink DSLG604T for example.
Alternatively (especially if you get a modem when you sign up) you could buy just a wireless router that has network ports. Eg- Airport extreme or Linksys WRT54G.
But since you already have an airport express, and you're happy with it's range and everything, AND assuming you DONT have or get an ADSL modem. You could get simply modem/router (not wireless). Something like the DLink DSL504T/G or Linksys AG241 (which all have 4 LAN ports). Then simply plug the airport express into that and you're off.
EDIT: the dsl502t and the ADSL2MUE only has one network port. You will obviously be looking for something that has 4 so you can plug the airport express and other computers into it. Sorry, my mistake. Click the model numbers on the linked ozcableguy's page and it will show all the stats.
ps- I have only got experience with the Dlink DSL502T and the Linksys WRT54G of the units I mentioned above. This is by no means a recomendation rather they were used as examples (though linksys and dlink are usually very good quality and highly configurable).
For more info on routers/modems/wireless, check out
OzCableGuy