If you're more concerned about Mac-specific threats on your personal Mac than with passing along Windows viruses to your friends, iAntiVirus is worth installing. It is free (for personal use), scans many files quickly, and isn't resource intensive.
Apple's latest Time Capsules are what any good networking and backup devices should be: easy to set up, manage, and forget about. If you're replacing an older base station and want to use Time Machine to back up multiple Macs, the new Time Capsule is well worth it.
The 2009 version of the AirPort Extreme Base Station packs in a ton of features, as well as speed, into a small, reasonably priced package. If you’re looking to replace your current Wi-Fi router, you can’t go wrong with the AirPort Extreme. If you’re running a small business, the new guest network feature alone might be worth the $179 price tag.