How to Check Your Mac’s Performance with Activity Monitor

Mac Snow Leopard offers an app called Activity Monitor, which is designed to show you how hard your CPU, hard drives, network equipment, and memory units are working behind the scenes. To run Activity Monitor, open the Utilities folder in the Applications folder.
To view each of the different types of use, click the buttons in the lower half of the window; The bottom changes to reflect the desired type. For example, if you click on System Memory, you will see the amount of unused memory; Click CPU or Network to view the real-time usage of your Mac’s CPU and network connections.
Keep tabs on Snow Leopard and what you’re playing.

Keep tabs on Snow Leopard and what you’re playing.
You can also display a separate window with your CPU; Choose Window → CPU Usage or press Command + 2. There are three different types of CPUs (the central processing unit, usually called the “brain” of your Macintosh) available from Activity Monitor:
CPU Floating Window: This is the smallest width of the CPU usage; The higher the CPU usage, the higher the reading on the screen. You can arrange the floating window in landscape or portrait mode from the Window menu.
CPU Usage Window: This is the standard CPU monitoring window, which uses a blue screen that looks like a thermometer. The screen works like a floating window.
CPU History window: This scroll screen uses different colors to help indicate the percentage of CPU time your apps use (green) and the percentage Snow Leopard uses to keep things running (red). You can use the History window to view CPU usage over time.
Do you have two (or more) bars on your CPU usage screen? This is because you are running one of Apple’s multi-core Intel processors. More than a drive under the hood!

Whichever display type you choose, you can drag the window anywhere you want on the Mac OS X desktop. Use real-time feedback to determine how well the system’s CPU will perform when running applications or performing tasks in Mac OS X. If this remains The scale is at its peak for extended periods of time while using a range of applications, the processor(s) operating at full capacity.
You can even monitor CPU, network, hard disk or memory usage right from the Dock! Choose View → Dock Icon; Then choose the type of real-time graph you want to display in the Dock. When you monitor CPU usage from the Dock, the green portion of the bar indicates the amount of processor time used by the application program, and the red portion of the bar indicates the CPU time given to Mac OS X.
Note, however, that seeing CPU capacity at its maximum does not necessarily mean that you need a faster CPU or a new computer.