i've always fancied getting a traverse adsl2+ modem - it presents itself as a realtek 8139 ethernet port internally, so no worrying about getting modem drivers to work (r8139 is in the kernel):
they even do a card that acts as an openwrt router with 2 nics and a modem.
i've used a sangoma card before (mainly because i couldn't get a traverse outside of australia!) and drivers are a nightmare - basically ported from freebsd to debian [badly!] stay away.