First of all Xpeed 300 PCI is SDSL modem and use Frame Relay encapsulation and you talk about ADSL and this days ADSL use ATM encapsulation
Second Xpeed company withdrawal its business in 2002.
Back in 2000 when we start to offer DSL in Egypt we use Xpeed 320r, Xpeed 320 Xpeed 300 and it was the worst hardware ever. (no wonder that company don’t exist any more)
So if ROS is support Xpeed 300 (I think ROS ver. 3 doesn’t support this card) that doesn’t mean ROS support ADSL PCI card
You can check
http://forum.mikrotik.com/t/support-for-pci-adsl-cards/17027/1
http://forum.mikrotik.com/t/can-router-os-aggregate-adsl-links-for-load-balancing/20567/1
http://forum.mikrotik.com/viewtopic.php?f=3&t=18762&start=0&st=0&sk=t&sd=a&hilit=adsl+pci
and you can vote for you request at
http://wiki.mikrotik.com/wiki/MikroTik_RouterOS/v3/Feature_Requests