ADSL Modem Goes Slower Than Dialup
Solution 1:
Not all ADSL modems are created equal. Here in the UK it is well worthwhile getting a modem based on an Alcatel chipset if British Telecom is your local loop supplier. Why? Because if you have a fault and your ISP gets a BT engineer to call around, they will plug in their own Alcatel based modem and if it works, you will be charged for wasting the engineers time.
I know your situation is different, but I would still recommend using hardware supported by your ISP. That way, if something goes wrong you can say you were just doing what they wanted, so it's their fault.
If you really want flexibility, the best option is to use the supplied router/modem as just a modem (most support PPPoE) and add your own router to do the clever stuff. This will allow you to keep the same router (and thus configuration) whatever technology your ISP uses (ADSL, ADSL2+, cable modem etc.). I've been meaning to do this for a while (with a Linksys WRT54G and DD-WRT) but I'm still wait for my delivery of the round tuits.
Solution 2:
If your first modem had no authentication credentials on it, some other device on your network was authenticating, or you don't have to authenticate in order to connect.
You will need to call your ISP to find out which condition is true for your connection.
Unless you're talking about an admin usr/pwd for the modem's management mode, in which case the difference is not relevant.