Try the AMR modem first, they are as good (or better) than the Winmodems, then if your unsatisfied with the performance, you can always ante up the additional $50. bucks or so for a good hardware modem like the USR's that have been suggested..........
Just make sure you get the AMR modem that was designed for your chipset as you may encounter problems otherwise..........
I'm using the: HSP56MR - AMR Modem that came packaged with my Amptron K7 830LM (ECS K7S5A clone) motherboard.
You can also install both the AMR modem and the USR's modem (no conflicts) ........... I have done this on my system, but have since gone to AT&T Broadband cable connection, so I just use the AMR modem as a back-up.