How to tell the version of your Mac OS

Step 1: Click the apple icon in the upper left corner of the screen

Step 2: Choose About this Mac

Step 3: The label right below the Apple indicates the version of this Mac OS, e.g. Version 10.6.2

Step 4: Right to the Processor label is the version of your CPU, from which we can tell the bits of the CPU

Here is a table to clarify the relationships:

Processor Name	32- or 64-bit
Intel Core Solo	32 bit
Intel Core Duo	32 bit
Intel Core 2 Duo	64 bit
Intel Quad-Core Xeon	64 bit
Dual-Core Intel Xeon	64 bit
Quad-Core Intel Xeon	64 bit

Ref : http://support.apple.com/kb/ht3696