Defense Against An Attack On My Home Network

I have reason to believe that my computers will be the target of an attack (small home network of 3 computers).

Since someone is specifically targeting me, am I correct that there is not much that I can do to prevent the intruder (assuming they have an intermediate knowledge of what they are doing)? All that I really know about the intruder is that with a phone number (and maybe a name) he or she was able to get at least the person's username for a site (perhaps they did a lot more).

If so, my objective is to soften the blow (of the attack) as much as possible.

I'd imagine that they are going to take 1 of 2 agendas when they break in:
(1) stealing information that they can use, such as bank passwords. Is this even much of a concern (what's my level of liability)?
(2) just messing up my computer. The core of this is destruction of files. My thinking here is to add more hard drives into the backup rotation. For example, with Time Machine (Mac backup program), I will have about 3 hard drives in my rotation. Rotate the back up hard drive every 3 or 4 days. This way, there should be at least 1 clean hard drive even if something sophisticated "hits" for several days before I notice.

My primary defense (may take some time to implement, and will be a headache), is to have a computer that is not connected to the internet and store the bulk of my files there.

I would like to still keep a good amount of files on the computer that is connected to the internet. I would like to find a software that monitors which files have been changed (or deleted) so I can respond quickly (perhaps every couple of days review a log to see if anything looks strange). Although it's not specifically built for this maybe I'll try desktop scout.

 

Use something like CryptoPevent *but* when applying settings use MAXIMUM PROTECTION. You will have to set it back to 'Basic Protection' each time you want to update a program that alters system files. It will prevent any change to any files. But if you are concerned about a hacker, you just need to ensure good general security.
http://www.majorgeeks.com/files/details/cryptoprevent.html
How to Protect yourself from malware!