Typically the ADSL modems are not supported by third party firmware. AFAIK DD-WRT doesn't have any ADSL modem support, so if you're going to flash your router with DD-WRT, it would work as a router, but the ADSL part won't work.
DD-WRT is nice to have if you want to do more advanced stuff, such as VPN's, custom firewall rules, QoS, multiple ISP accounts, etc.
I really won't recommend installing OpenWRT/DD-WRT on an ADSL modem, unless they tell you explicitly that the ADSL modem part will work and that the installation process is straight forward. That OpenWRT link that hereticangel provided looked like the guys sacrificed their ADSL modem part.
