Two technicians actually came out and we were very nice and helpful. They confirmed our connection stunk. They put in a "line filter" (whatever that is) and they switched out modems. They then tested the connection and it was once again blazing fast, like it used to be. Everybody was happy. Way to go Comcast!
Crystal pointed out that there was also the possibility that some of our other equipment could be slowing down our connection. Our topology is: cable modem -> Vonage modem -> wireless router -> computers. Our home built desktop system has a wired connection to the router, even though it actually has a wireless PCI card in it as well. My MacBook uses the wireless of course. Just to be sure that the other stuff was not causing any problems, I did some experimentation.
I did speed tests with various configurations. First I tested with the cable modem connected directly to the desktop computer. I used this as a baseline. I got 600K down and 550K up.
Next I put the Vonage modem in between the cable modem and the desktop computer. This had no significant effect on the speed. It was actually slightly faster down and slightly slower up. No difference. I was happy to see this.
Finally I set up my usual setup. Now I found a dip in speed. I was only getting 500K down and 475K up. It was a pretty consistent 15-20% drop in speed.
Now 500K down is still plenty fast. It's certainly fast enough to support Vonage and "heavy" internet use like streaming video. I suppose some loss because of the router (a D-link DI-60 80211g that includes a hardware based firewall) should be expected, but 15-20%? I really don't know if that's a really bad amount or just par for the course. Do I need a new router?