Mike, 70 dollars for a 128mb stick of sdram is a rippoff and you should go elsewhere and get it for less than half that price. Don't pay any attention to them saying anything about voiding their warranty, just make sure if you need to use their warranty for anything to take out anything you replace and put back in their components, and make sure there is no sticker and if there is make sure you can re-affix it to the case.
Sometimes they put a sticker where the case comes apart to tell if you have worked on it. If you want to be really slick about it you can call them and tell them that your modem doesn't work at all, they will likely tell you to open your case and reseat it if all else fails. There you go, they opened the case for you, therefore making it so you didn't void it by opening it, but that situation is a last resort. Just install the memory.
As for what memory you need, you can basically probably use any old pc100 or pc133 cas 2 or cas 3 memory. pc133 cas2 memory is the best and pc100 cas 3 is the lowest performing. Second best choice would be pc133 cas 3 and you do not want anything that is pc100 so if it has that in it's description you don't want it, you only want pc133, preferably pc133 cas2.
Good Luck
John-Paul AKA veryape