If you're looking at only backing up certain folders from your four computers then Drive Image is probably not what you are after. Drive Image will make a complete image of your hard drive including your windows installation and everything else on the drive. This is good if you want to do a complete restore but it does take up a lot of space and makes keeping an up-to-date copy of your critical data very difficult.
Admiral Michael mentioned a program called Second Copy earlier, and I think this would probably be the best option you could use to keep an up-to-date copy of all your critical data. You can download and install the program onto one of your existing computers (ie costing nothing). You'll then be able set the folders that you want backed up from each of the other computers and you'll also be able to set a schedule for it to backup automatically. Using this method you could have the program backup your data once a day (or even more) so that in the case that one of the other computers has a complete hard-drive failure you'll still have all of your data intact and the most you'd loose was something you made since the last scheduled backup the night before.
You can even set the program to move any deleted files into an archieve folder so that anything that you have had backed up that you delete is not gone forever but conveniently waiting for you to recover on the computer with Second Copy. (if you use this method you'll need to clear out the archieve folder every so often so that you don't run out of space)
If you want a more efficient solution you could build an inexpensive server (which is nothing more than a standard computer) and install Second Copy onto that. This way you could leave your "server" on all the time and any computer which is on at night you could schedule the backup to run then, and the other computers to backup during the day. Depending on the hardware you select for you server would also depend on whether it could host games, as a lot of today's games have some really demanding requirements. Farcry for instance recommends a 2GHz processor for a multiplayer server.
You could use the server as a firewall if you wanted, you'd need to have two network cards installed on it (one to connect to your other computers and one to connect to the internet) and then it would just be a matter of installing a software firewall. You wouldn't really get that much advantage, except you wouldn't have to install the software on each of your four other computers, only once onto the server. The setup could be convenient however if you wanted to monitor how much you have downloaded each month for instance, as you could set up monitoring software on your server and it could tell you the combined downloads of all four computers.
Patrick
