PBX host card for POTS at home and VoIP to the outside, ISDN client card for fax, and two RAID controllers to tape all the calls!
Of course, buying a PCI card now will lock you down in the future, because you'll always need to buy a board with PCI slots, to carry over those cards. But PCI is pretty much dead, so you deliberately limit your available choice of mainboards.
Just leave those slots free, and be happy you won't run out of interrupt bandwidth.