Sometimes when you need to download and install software you need to choose between 32-bit and 64-bit. I can never remember what my system is, so I use this method to find out.

Execute the following command in a terminal

pg /proc/cpuinfo | grep "model name"

This will output the model name of your processor. For me it output "Intel(R) Core(TM)2 Duo CPU P7570 @ 2.26GHz".

Then I googled the name and found a page describing the processor. Under "Instruction set" is says that my system is 64-bit.