Ok, all the New USR internal's are Winmodems if I remember correctly. Here's the problem with winmodem, cant stand on its own. No UART chip DSP chip, needs the processor and OS to function correctly. Which also means uses up some of your processing cycles there by slowing down your performance to a certain degree.
In my personal opinion it is always best to go with a "real modem". A hardware controlled modem, ISA is just fine for a modem though.
The Zoom DualMode 56K and Diamond SupraExpress are talked up a lot but I've tried both and wasn't impressed. I have a 56K Aztech PCI a got online for $42 shipped that I thought was better than either. But my recommendation would be a 56K ISA US Robotics OEM (NOT a Win Modem version) widely available for under $70 if you skip voice. They dial and connect fast, have that nice loud speaker, and connect very reliably.