It's best to check the specifications of the kit you're buying before throwing out the supplied modem though. Generally speaking, if the device is labelled ADSL Router/Modem, it will be okay to replace the supplied equipment. However, if the product is not listed as a router or modem, the hardware may lack the ADSL de-modulation circuitry. Most gateways tend to be configured in a way that they just handle TCP/IP packets, and don't modulate/demodulate. That said, they are great devices, usually having NAT, DHCP, DNS, Firewall, and sometimes wireless all in one neat little box. Also, check with your service provider regarding replacing the supplied modem/router as they may have restrictions in place to prevent this.
IMHO your best bet would be to stick with the supplied modem and put your beer money into a cheap gateway.
BTW, although most of these gateways have a 3 or 4 port hub in them, they will usually take extra hubs/switches being connected to them. The maximum number of PCs is usually restricted by the software (usually the DHCP server).
I got a bit carried away there. Hope this helps anyway.