Be careful
Paul
Hope this is not too late. It is NOT straightforward to work the answers out for going ADSL, so don't be embarrassed - we've all been there. While this maybe a great forum, is is open to the rest of the world and everybody has different standards (eg DSL vs ADSL - these are different).
So I would listen to Aldo's advice. GO TO THE ADSLGUIDE site and subscribe to the Apple discussions forum. This is a great site for UK ADSL Mac users.
I would also go further than some here:
1. On NO account get a USB modem. While they can work with Mac's they require drivers to work. Drivers cause problems, especially when OS's change - like going to 10.2. USB modems are popular because many PC's still don't have ethernet. Pretty much every Mac in the last 10 years has had this built in and this is the way to go. There are many other reasons to go for an ethernet modem/router which you can find on the ADSLGUIDE site, but this is the most important. (I use a modem/router with a wireless hub and have instant-on broadband anywhere round the house with my Powerbook).
2. I WOULD personally always get a modem/router. This will give you more flexibility. Costs of basic ones are no more than about £60 - again check the site - there are loads of reviews, and ask questions there.
3. As for ISP's this is where I would differ with Aldo. Pipex may be a reasonable bet, but there are others too, some of whom are better with Mac's. I personally use Metronet.co.uk which starts at £10 per month (yes, £10 per month) and has worked brilliantly for me. However, Zen, Plusnet, Easynet etc are all reasonable for Mac users I think. Just avoid going for one which throws in the USB modem as part of installation - get your own equipment.
Hope this helps
Ian