Hi, Amie.
I'm always happy to add to the discussion on security. I had a bit of a shock a few years ago at work when seeing how the usual "out of sight, out of mind" attitude didn't negate the fact that there were often attempts where people tried to get into our machines (which weren't Macs, by the way).
I think spyware is not such a risk right now, but, if you're concerned, have a look at the
SecureMac.com web site, and, in particular, you can try
MacScan.
Not having ports open is a good thing. As a general rule, if you don't need a service (eg. FTP, ssh, etc.) don't switch it on! Having your firewall switched on is also a good thing. If you want to test what your computer looks like to hackers, you can visit sites such as
Shields UP!, although there are others too. Also, if you can use your machine behind a router, preferably one that does network address translation (NAT) and uses port forwarding, that would be even better.
The NIDS programs listed above are basically like burglar alarms. They're not necessarily about spyware, they're about looking for important files being changed in a way it shouldn't be, or signs of people trying to get in.
If you're really concerned, it is also possible to restrict which users are allowed access to your machine, what they can access, and which IP addresses they can contact you from. This all requires a bit of fiddling using the Terminal, though, so it depends on how happy you are doing that.