There are several ways to know the architecture of the Ubuntu you are using. I prefer “uname” command to find it out.

Type in the terminal: uname -a

Output: Linux desktop-name 2.6.38-8-generic #42-Ubuntu SMP Mon Apr 11 03:31:24 UTC 2011 x86_64 x86_64 x86_64 GNU/Linux

Conclusion: x86_64 (or i686) denotes 64 bit and x86 (or i386) represents 32 bit.

Extra: To know the processor: “uname -p”

To know the hardware platform: “uname -i”

To know machine hardware name: “uname -m”

In all the above cases: x86_64 (or i686) denotes 64 bit and x86 (or i386) represents 32 bit.