External modems are the equivelant of a SCSI device.
PCI modems are the equivelant of ATA66 IDE HDDs.
ISA modems are the equivelant of ATA100 IDE HDDs (in terms of ping time and relative speed)
winmodems, any kind, are the equivelant of a 5 1/4" floppy drive.
ZooM and USR are the best, I've been using a 56K ZooM ISA for the past 2 1/2 years. Got it for free, and works great.
-SW