I'm using a broadband router with a firewall. This is what I'm using and what I would recomend. It will be the easiest to use and set up. You can use one IP address for all of your machines so you won't incurr any extra charges. You might have to let the tech install the modem on one machine then set the router up yourself later. You might not get support from Knology for such a device so you might have to hook up your computer straight to your cable modem if you have to call tech support.
You can use a simple switch or hub between the cable modem and network but then you will need a unique IP address for each computer and they will charge extra for each address. Plus you would have to set up more security on each computer.
I would suggest you check out the networking and security forums for more more detailed advice and up to date info on equipment and software. There is not really anything unique to Knology with these issues. You might also want to look at software such as ZoneAlarm for additional security. These programs can control which programs on your computer can send out information, this is useful for keeping spyware from sending out info you don't want to get out.