A NIC card on its own (LAN card) will not work and the advice you have been given re a separate ISDN card in a PCI slot is valid.
If you only have the one computer then why not have the box near the computer and use the supplied USB connection?
Might you have a network of computers on a LAN?
I have a network where all of my computers feed to a hub which in turn feeds to an ISDN router, the Netgear RT338 which has served me well over the last couple of years.
Because broadband is taking off there are quite a few ISDN routers on places like EBay where the original cost of £140+ is more than halved.
The Netgear 348 has a built in hub to accomodate 4 computers.