Just to add to the confusion I guess:
Diamond Supra PCI internal was the best modem I've ever had. I had solid connects, fast downloads and no problem whatsoever for just under 30 quid. Then I read about winmodems being the source of all evil and the opportunity came along to swap it for highly recommended Hayes Accura internal thing (full blown modem, not using CPU - blah-blah-blah...).
Well - connection is fractionally slower (so are downloads), it's stable... Err...
Was it worth extra 40 quid? Definitely not. Absolute waste of money.
External modem is better? How? Your ISA/PCI thing is sitting right on the data bus seeping the power of the mobo and feeding the data straight in and out. Any external is the same inside as internal with the only differences being a separate box, blinking lights (yawn), and a power source. And, of course, few more cables like you don't have enough of them. And let’s not forget an extra cost too.
I disabled both serial ports in BIOS cuz there's absolutely no need for them to be there - it's a legacy from good ol' DOS days. Now I have (gulp) spare (!!!) IRQ's and no device conflicts.
The worst modem I've ever had was made by USR. It was a winmodem that windows refuse to detect, drivers kept crashing and god forbid if you got it working you dare change any settings... After 6 months of hell I just put it in the bin and swore never to touch USR stuff again.
YMMV
p.s. ISDN (in the UK) - what a joke! My gf is on this crap paying a small fortune and downloading at 6-6.5K tops. Me and my boring phoneline do it at 5.5-6K

I guess
It
Still
Does
Nothing is still a pretty good description for it

.