I just put a Zoom 3025 in a customer's computer. Internal, PCI-slot, not a winmodem. Worked okay. Although I had to drive back down to install it for her (she bought it and tried it herself). PCI modems will be...quirky. Recommend a slot in the middle...because I can. Didn't seem to work in the 1st PCI slot. Tried fixing the drivers and whatnot, eventually completely removed them, reseated the board in the 2nd slot that I tried and voila. Also plugged the phone lines into the proper ports on the back.
Connected at 40.0 kbs. Not too shabby. Worked alright on less graphically intensive pages.
Althoug, I don't think a winmodem will be too bad nowadays, as obviously CPU's are little more powerful than they used to be, and you wanted all your horsepower going into programs. Not too mention, most computers will have memory available now and not have to use virtual memory as much.
As a random guess, if you have a p4 or athlon xp and 128-256 MB ram, you should be okay. Personally, I would say to get an external modem, because that way you'll have blinking, flashing lights...
On a side note, it was really bizarre to a modem dialing up again....
Edit: Apparently the Zoom is a "host-based, controllerless modem." Which apparently means it's somewhere in the middle. Passes a little work on to the CPU, but does most of it itself. I guess....