Getting a new DSL modem will not speed up your internet. Somewhere far away at the isp, they have a computer with your phone line settings, and in thoes settings they have a little meter that tells your internet speed. On the side of their compuyer, there is a slot that takes $20 bills. The gurus that work there get all the mail with the payments, and open each letter and feed the money into the slot for your computer. The more money that is fed into your section, the higher the meter reads and the faster your internet goes.
I tested this theory once when I took my DSL modem to a freinds house where he has a slower service than i do but the same ISP. I used my modem and user name and password, and connected at the same 320Kbits that he got with his modem and password. (my service is 4.5 megabits.)
I later asked another friend who is a works for nortel, with internet and phone technologies and he confirmed that the speed of your internet service is not controled by the modem or your username and password, but by the isp where they set the limits on your phone line. However, i didnt metion the money slots and gurus, so i cant prove that part.