Unless you have a fast cpu, a pci modem will work like suck, unless it's a full hardware processed modem.
If your board is new and it doesn't have isa, then chances are that you have a PIII 500 or better cpu and just about any pci modem should be ok. If you want something that doesn't take alot of resources from the cpu, don't go with a winmodem, go with a full hardware internal modem, which will cost you more than the cheaper winmodem, but it's worth it. Better yet, go with an external modem and you should be cruising(as fast as a modem can go anyway heh) cause they have their own hardware processing.