I've had a motorola Surfboard modem on Comcast for a few years now. Ross Carlson gave me a tip about getting better speeds if you buy your own modem instead of renting from Comcast. After installing my own modem, my speeds jumped 5x!

Check the chart below. I went from getting 3900kbps consistently to 13500 to 22500kpbs. The rogue low score in there was done while streaming Heroes in HD. So even while streaming HD I get better throughput than before! awesome!