the only way to find out if it will work at all with AT&T is call them and ask if it is compatible with there system.
personally I avoid Modem/Routers when ever I can. I rather spend the extra money and have a modem and separate router
Although yes, a great answer, we don't live in an honest world. Companies like AT&T may not list this as something compatible with their service on the sole reason that they don't get kickback for selling/recommending them.
Note if your not tech savvy, make sure to use the cd that came with the Linksys device. It should walk you through step by step on how to set it up. There is at&t's tech support number at 1-877-722-3755. You may or may not need to know a username and password combo if its a pppoe setup.
To emeralds point though, I 100% agree that its better to separate the modem and router. Although you may save money buying a modem/router combo, I would think about replacement cost as well. If your combo breaks, you either end up forking that $100 (according to bestbuy.com retail price) vs say if the router breaks its a mere $50 (for say a linksys e1000 router). Plus, sometimes you don't really save money as long as you look carefully. Example, a Netgear ADSL2+ modem retails at $50 at Best Buy. Combine that with a $50 router, you're still at $100.