Actually, your question is much broader than the title says it is. It involves not only security as in protection against malicious intent, but also protection against things you do not have influence over (i.e. fires, earthquakes, that sort of thing).
I think it would be best to handle this 'bottom up' (or top down, depends on your point of view), so to say, starting with the computers themselves, then going to your own network and ending with your computers on the internet, as it were.
Your computers are actually the most important asset. They contain the data that needs protection, so I'll talk about them first.
Backup, or: keeping your data from harm
The first thing to do to keep data safe is to implement a proper backup procedure. This ensures safety of your (their) data in the disaster-protection/recovery sense. What a proper backup procedure is, depends entirely on your budget and the sensitivity of the data you work with. A good starting-point would be an encrypted and off-site backup done via the internet.
Firewall, or: keeping others away from your data
Second would be to look at the protection of the computers in the firewall kind of way. Any computer needs a decent firewall, but there is a choice: you can firewall your individual machines, or you can do what is called 'perimeter firewalling'. I would suggest you choose the third option, a combination of both.
The reason for doing both perimeter and workstation firewalling is simple: a perimeter firewall doesn't stop your workstation getting infected via email-attachment. Your antivirus product might, but a workstation firewall should ensure it doesn't spread via the network to any other computers.
Antivirus, or: keeping data away from your data
Well, the subtitle is slightly weird. That's because this actually could have the same subtitle as the previous header. The third step to protecting the data on your computers is to make sure no data (i.e. programs) with malicious intentions can run on your computers. To do this, of course you need an up-to-date browser as you mention. But you also need a way to inspect what is what and what is and what is not malicious. In other words: you need a virus scanner.
Use the 'net to find a decent one, or, even better: buy a corporate version. From what I've heard McAfee is quite decent, as is Kaspersky.
Of course, you can install any security software you want, if the configuration isn't right, it'll do you no good. So make sure you check the settings. For firewalls I'd recommend to basically shut down everything. Nothing goes in, nothing goes out. After that, start to open ports on a need-to-open basis. You will want port 80 going out, for example, because that is the port HTTP uses by default. There are a few others and the internet knows them.
The other thing is choosing decent passwords. The internet has plenty on this, too, but rest assured that it's quite important.
Now that your computers are essentially secure, it's time to secure your network. It's important to know that any encryption or safety can be breached, given enough time and resources. Also: a possibility that is not there, is no possibility. That sounds obvious enough but it's very important.
There is not much to secure about a wired network, to be honest. The most important thing here, in the network-sense, is the perimeter firewall. You can of course do all kinds of neat obfuscation like not using DHCP and using a different private subnet than default (192.168.1/24), but in the end they are not a security measure, they just make it a little bit harder to find out what you're doing.
But then there is also the physical way of viewing it. Security on your box is no good if I can access it physically. Same goes for your network. I take it it's inside your home, but it is still a thought.
The wireless network is much more of a PITA. Unlike the wired network, you don't need a physical plug to access it. So it stands to reason that the security must be that much stronger. Fortunately, 'they' have thought of this and given us wireless encryption.
In the beginning there was WEP. It was good enough at the time, because no-one could crack it in reasonable time. Currently, it takes about two to five minutes to crack it, on a reasonably busy network. So WEP is out.
Then WPA came along and was much better. However, the WPA standard is already under jeopardy. The advent of the GPU as a calculation processor instead of doing graphics only has sped up key-cracking a humongous amount. So WPA is also out of the question.
At this moment, only WPA2 can be expected to offer reasonable security, especially if implemented with RADIUS (and not PSK). This is where the security versus resources comes into play. If you want more security, you will have to install a RADIUS server. This will come at a cost, however, so the added security might not out-weigh the added cost.
I think the best you can do for now is use WPA2-PSK with a sufficient passphrase. Make it long, make it complicated.
Anything like not broadcasting your SSID is bogus, any tool can find your network anyway. So is registering MAC addresses. It takes about a minute to find an accepted MAC and set my adapter to it.
- Good passwords and pass phrases.
- Offsite backups
- Firewall your workstations and perimeter
- Upgrade wireless encryption to at least WPA2-PSK