My router (netgear) also lets me name the computers I want to give access to, in addition to adding their MAC addresses, and it gives access to them and only them. If you've run MS Home networking, it'll get the computers named and you can go from there.
Doesn't your router have plugs for network cables in it? You can use that and disable the wireless feature instead of buying something new. Or you can use the built in Windows Internet sharing, its not the greatest, but it works and will at least help you troubleshoot the problem.
What about calling tech support for your router? You know how people used to steel cable... Maybe someone has figured out how to steel cable internet and the problem is happening before it even gets to your house?!