How to become my own ISP

I share my internet with 5 other computers, when a computer reaches 5GBper
month I want to be able to Stop them downloading or shape them
automatically.

I have 2 ADSL2+ Modem/Wireless 4port Router and a switch, 2X PCI wireless
cards and 2X USBv2 54mbps

Any ideas off what i need or configure????????? please.

do i need a server or just a simple hub or switch??????