PCI is convenient if you have free slots. No extra outside. You can get a lot of cheap ones from ebuyer,click here for examples PCI one £27 click here, usb one £18 click here. You also need microfilter for each extension, cheap one cost about £1 click here, quality cost more. USB one use more comouter resource and need external power adptor, so PCI one is better.
Just found you can get PCI one cheaper from click here, they also sell quality ADSL filters. It is better to get good micofilter otherwise your phone communication quality will be affected.