I've setup my spare laptop with a fresh FC4 install. Setting up the modem was a piece of cake and while I did all the setup with vi and the command line, I think it'll be possible to get it working through the GUI only for those who fear the command line.
Speed tests aren't going too bad. The throughput definitely fluctuates. Earlier I downloaded firefox onto my wife's computer at 100KB/s but right now gaim and thunderbird are coming in at a combined 20KB/s. Not anything close to a speed record but a lot better than the tin can and string we were using earlier this week.
Vonage certainly works. I placed a couple test calls between my cell and the vonage phone and the audio quality was very good. If you try it you'll get better results if you turn the bandwidth saver down to 30kbps via the Vonage site.