iStat is a great way to know what's up
Two friends recommended that I take a look at a utility called iStat Menus from iSlayer. It's a nice little widget that allows you to see that status of your machine.
While it can monitor a lot of different functions, the two that I cared most about were Memory and CPU Usage, so that's what's plugging away up in my menu bar.
This actually helped me solve a little problem I was having. Normally my MacBook is whisper quiet. All of a sudden one day my fan started really working overtime. I wasn't sure what that was all about until I noticed that my iStat menu indicated that my dual core CPU was running at a sustained 50% utilization. That's not good - I wasn't really running anything important at the time. I slowly closed each of my applications, watching iStat to see if the CPU usage dropped and, sure enough, it was iChat.
Not exactly sure why iChat was causing the memory usage but at least I knew what was up. I'm sure a Mac expert will tell you that there are system level ways in which to identify where memory is being sucked up without having to get a utility like iStat but this is a really nice little utility and all they ask for in return is a donation of your choosing.