HFS/HSP/HCF modems are inexpensive and generic, and if you're getting 40kbps, you're probably doing well.
Cheap modems from Conexant/PCTel use your computer's CPU for some of the modem's DSP functions, meaning that if your computer is at high CPU usage from other apps, modem speeds and transmission quality may suffer, or the modem may even drop connection. If you have to use a modem and want top quality, you'll need to spend the bucks and get a hardware DSP modem, such as the 3com/US Robotics Pro, or an external such as the 56k Sportster or Courier V.Everything. Even when these don't connect at top speed, they're much more resistant to line noise than others; extensive testing has proven it again and again.
Intel Q9450 @3.2GHz, Gigabyte GA EP45-UD3P, 4 x 2GB G.Skill @4-4-4-12
Antec P160SW case (modded), Xigmatek 750w PSU
3x 500GB (RAID-5), , OptiArc 7200S, ASUS E818A3T
Creative X-Fi XtremeGamer, Hauppauge HVR-1800, Radeon 4890