Beg for an ethernet modem. If they're your only option, and are uncooperative, hit up
DSLReports/BroadbandReports's forums, and find out what ethernettable hardware works.
If they use PPPoE, you may want/need a standalone router (Linksys/D-Link/2Wire box) to handle that side of the equation, or a copy of Miami Deluxe.
All that done, comes the question of how to get ethernet on the Amiga. X-Surf is an obvious possibility. As the owner of some erstwhile 2000s myself, I like the idea of a USB controller (Highway, Thylacine, Algor) and a supported USB ethernet adapter. This gives you a good balance between price (as far as I know, $Thylacine + $USB_ethernet_off_Pricewatch = $X-Surf), performance (pre-Algor, speeds may not reach peak ethernet performance, but should be plenty for DSL), and future expandability- so far as I know, USB is as close as you'll get to PCI on a 2000.
Of course, if you can afford it, the Highway/Algor + Norway is certainly an excellent (and less waiting-for-shipping-from-Australiaey?) product, it just doesn't come out
Not sure exactly what the deal with USB stacks is (no offense to their authors!), so if you go this route, doublecheck the value-propositions in terms of any additional licenses needed.
...sadly, even if you have a USB card, it's quite unlikely that you'll be able to plug in the USB modem directly and play. While the current USB modem hardware might be becoming more standardized (I've no idea, really), free *NIXes have barely caught up, let alone Amigas. Maybe that'll change for OS4, but it's a headache best avoided for now.