Jimmy Macintosh,
Given that you have an RJ45 ethernet port on the existing cable modem, you will need an RJ45 cable to connect it to the next device in your home network.
This next device, at its simplest, is a hub (old technology) or switch, whose function is to allow all the devices on it to physically connect to each other (a star topology). So all connected PCs can be made to communicate with each other, and to anything else connected - in particular, the internet connection.
In general, this device is a multi-function appliance - it will contain several of the following functions: Switch, Router (to join the home network to the outside world - at a logical TCP/IP level), Firewall (a router with enhanced security functionality), Wireless access point (WAP), print server, USB disk connection and possibly others.
What you need is a combination of Switch - to connect your PCs together using Rj45 cables, Firewall (and router) - to link to and protect the PCs on your home network from the internet, and WAP - this functions similarly to the switch, connecting PCs using wifi to the home network.
This unit will have an RJ45 uplink port to connect to the cable modem, at least 4 switch ports to connect to your PCs, plus the WAP antenna. The WAP is in fact a hub, internally connected to the switch - in essence the WAP has an "uplink" connection to a built-in downlink port in the switch.
There is no security between the wired and wireless parts of the home network - access to one gives full access to the other. So implementing WPA or WPA2 is essential.
Do bear in mind that having access to the network is NOT the same as having access to the PCs on the network. But it is a necessary first step!
Note again that securing the wireless network is not the same as securing your PC from internet attack - that is the function of the firewall. You presumably have only got a software firewall (if any) protecting the PC from internet hacking - the addition of a hardware firewall will significantly increase your network defences!
I'm considering making it possible to connect lot's of different items to this connection, standard computer, webcam (watching new pet at home for a little while until it settles in), HD DVD player and allowing guests to use their laptops.
Connecting your computer, and guest computers (that you allow), to each other and the internet is primarily what the home network is about. Connecting other devices is possible, providing that they already have the means (physically and functionally) to be network-connected. a WAP or switch will not provide the means to connect non-network-aware devices. Of course, if they can be connected to a PC, which can in turn be connected to your home network, then that's a solution.
For what its worth, I recently replaced a Netgear WAP/switch/firewall that was locking up daily with a Linksys device, and it has been performing perfectly for over 2 months, without a hiccup.
I think the problem was that the Netgear device was originally WEP only, with a firmware upgrade to WPA, and the firmware upgrade was errr... cr@p.
HTH,
SD