I think you may have a misunderstanding of the technologies at hand. If you have a router, when it talks to the rest of the internet, the rest of the internet sees the public IP address that comcast leases to your modem. When you connect to the router, the router in-turn leases you (and anybody else connecting) a private IP, usually starting with something like 192.168.xx.x
Each person connected to the router gets a seperate private IP address, but to the rest of the internet, any requests sent out by anybody looks like it's coming from the public IP address. That slowdown (i'm asuming everyone is connected wirelessly) is because the bandwidth to the router is saturated. There isn't really any way to avoid that.
To make matters worse, is if your Airport is running in G mode (54mbs) and anybody that only has an A or B wireless card connects, the whole thing slows down to accomodate their speed. If that is indeed the case there is a great way to solve this. Get an inexpensive router, maybe one that is only compatable with the B protocol. Connect the AP Express to one of the router's ports and connect the router to the modem. Then configure the Airport to only accept G connections. Then make sure the cheapo router is not set to auto channel mode. Set it to either 1, 6, or 11. Then set the airport to manual channel mode as well, and pick a different channel from 1, 6, or 11. Those are the only three channels that do not overlap. That way, you are not bogging down your 54mbs traffic, whilst still allowing A and B traffic to take place.
Contrary to popular belief, your Mac will work on other routers besides the Airport Express. I'm connected to a Buffalo WHR-54GS. Works great!