Tighten things up a bit??!!
To hack your network I would have to spoof my Mac address and sit outside for however long it takes to gather roughly 2 Gigabytes of data from normal network traffic and then a further however long cracking it. Tighten things up? Well, sure, you can narrow the ip range as your router will scale up from its own (class D) address. Therefore your router will probably be seen as 192.168.2.1 or something like that so if you have three computers then you could safely set the range from .2.2 - .2.8 or something. Make sure you have a password set on your router, upgrade to WPA change the default ip address of your router, change the name of your network every week, read this book:
edit: DHCP will not necessarily set .2 and .3 as your computers ip addresses. Use static addresses to get around this ie. set them manually