Depending on your router, you can hack it and put custom firmware on it, and increase the power of the signal. Down side it will also increase heat, so make sure it is in a well ventilated area, basements are usually cooler then the rest of the house so that will help.
Also horsefever I am betting your wireless is faster then your internet, unless your are paying $100 or more, or live out side the US.
But to answer your question, How I think it works I am not sure, it depends on how you ISP has set everything up, Some do it on a account base (i think) so the modem will auto log in to there system, giveing your access, and setting limats on how fast and how much your can transfer stuff., Others like comcast, it based on s/n numbers or something on the modems. but I am sure ether way you can only connect one at a time or only one period.
If I were you, I would look in to upgrading to Wireless N giving you 150mb to 300mb over wireless. If your clients don't support N I would get a N bridge, the clients can directly plug in to.
<to Tigsounds> I like that Antenna by the way