Don't bother removing your SSID from broadcasts - that won't stop anyone who really wants to from finding your router, and it does nothing to stop them from connecting to it.
To protect data sent over your wireless network, here are your options:
1) The strongest form of security available on wireless routers today is WPA2 using AES encryption only. If your router and your clients all support this option, use it.
2) The second best form of security on wireless routers today is WPA2, allowing either AES or TKIP encryption. If you have a client that can't do AES (a limitation of some older devices), choose this option.
3) The third best form of security today is WPA using TKIP only. If this is the best your old router can offer, use it.
Any router purchased in the last 6 years will support one of these three options. Only really REALLY old routers are limited to WEP - using this option is NOT recommended. And of course don't turn wireless security off entirely - this "open" or "none" option is available in nearly all routers for situations where security isn't offered (like most commercial Wi-Fi hotspots).
Now, what about making sure that no one else can use your network?
1) The strongest form of access control available on wireless routers today is 802.1X, also referred to in products as WPA2-Enterprise or WPA-Enterprise. But this option is mostly used by businesses because it requires a RADIUS server to log in each user. Some routers have an internal RADIUS server that lets you configure a list of users and passwords so that you can use 802.1X. If your router doesn't, keep reading...
2) The second best form of access control on wireless routers today is PreShared Key, also called PSK, WPA2-Personal, or WPA-Personal. PSKs are a passphrase known to everyone that you want to use your network. Anyone who knows the passphrase will be able to enter it when prompted, usually the first time they connect. Anyone who doesn't know the passphrase won't be able to connect. PSKs can be guessed or "cracked" if they are too short or too simple. You're pretty safe if you use a PCK containing at least 20 characters, including some random text that's not a word and is hard to guess. Don't make your PSK your kid's names, your pet's names, or your favorite team's name. Do make your PSK complex - for example, 4goodNE55sakeTHSizl00000ng.
3) As an extra measure (or last resort) most wireless routers let you allow access by specified MAC addresses and deny access by unknown MAC addresses. This is also called a MAC ACL or MAC filter. This is NOT recommended because anyone can set their own MAC address to someone else's, but it's better than doing nothing at all to limit access to your network.
In your case, your Belkin 54g supports WPA2 so make this easy on yourself:
* Choose Security Mode "WPA/WPA2-Personal (PSK)"
* Choose Authentication "WPA2-PSK" - this sets Encryption to AES
* Type a strong PSK into the box
* Try to connect from each PC or other client you want to use
If you run into a problem, try choosing Authentication "WPA-PSK+WPA-PSK2" instead. Reboot the router, delete any saved "Preferred network" on all PCs, and try again.