well...
there are several types of DSL...and even ISDN often is (imho wrongly) referenced as (I)DSL. The common DSL line however is ADSL, the A stands for async.
Async, because the downspeed is higher than the up(load)-speed. (Which is ok for 99% of internet surfing anyway).
It depends where you get ADSL from. Ameritech (as far as i know) offers DSL with a downspeed of 768KB/S and an upspeed of 128kb/s. (correct me if i am wrong). Just picture that a modem has 56kb/s..and normal ISDN has 64/Kb...so you know that the upspeed is around twice as fast as your modem and the downspeed around 13 to 14 times faster as a 56k modem.
There are other DSL companies which (often) for the *same* price offer 1.5 MB/s (twice 768kb) downspeed (and either 128kb/s or even 256kb/s) upspeed...meaning for the same price as ameritech you get twice the download speed and upload speed. A good site for checking availibility for DSL is
www.dslreports.com (
www.dslreport.com ????).
THEN there is SDSL.. its NOT so common and usually much more expensive. S = synchron. Meaning: The upload and download speed are the same...eg. you COULD find a provider offering 768kb/s downspeed AND 768kb/s upspeed - but this usually is very expensive and more meant for businesses, servers etc. where its important to have bigger speed uploading.
ISDN (or ISDL) usually is 64KB (a bit more than a modem) down AND the same speed up....and two channel ISDN is 128KB down and 128KB up....
Anyway...to come to an end....if you look for DSL find a provider which offers 1.5 MB/s down and 128k (or 256k) up...and dont let you fool paying the same amount for a only 768kB/s downspeed from eg. ameritech or something....or let you fool by getting ISDN (slower) because ADSL is not available in your area because of the distance to your phone company - many companies do that if real ADSL is not available and try to sell ISDN. (Which is way expensive here in the US btw. compared to europe....in germany eg. if you order a phoneline you automatically get a 64K ISDN line..and here they want an arm and a leg for normal ISDN which is ridiculous