Maybe I was fortunate to have everything go REALLY smoothly when I networked my house, but you'll probably have a simliar experience once you get the info. you need. It was really easy.
Here's the instructions I followed:
Plug all computers into the router (or connect via wireless cards) and turn all computers on.
Turn on Personal File Sharing and Windows File Sharing on the Mac.
On the PCs, setup shared folders (at least 1 folder on each computer).
To do that, right-click any folder, click the sharing tab, and enable sharing of the folder by enabling the box next to "Share this folder on the network."
Next, right-click the My Computer icon on your PC desktops. Select the Computer Name tab and give the computer a unique name you can use to identify them, and will know which one you're connecting to.
Save that and you should be good to go.
To connect to one of the PCs from the Mac, go to the Go menu and select Connect to Network. You should see the PC names you just gave them.
You can also access the PCs by typing smb://ComputerName (the name you gave them).