Is my Ubuntu install 32 or 64-bit?
Go into the terminal and type in.
uname -a
If your results are similar to the one below, then yours is 64-bit; otherwise, it is 32-bit.
Linux ubuntu-tm 2.6.35-28-generic #49-Ubuntu SMP Tue Mar 1 14:39:03 UTC 2011 x86_64 GNU/Linux
If you have the x86_64 then your machine is 64-bit.
If your results are similar to this one; then you have 32-bit.
Linux Server 2.6.15-23-386 #1 SMP Tue Mar 1 13:49:40 UTC 2011 i686 GNU/Linux
Notice the i686 in the code, that means your machine is 32-bit.
UPDATE:
Type in the following in the terminal;
uname -m
It will give you either x86_64, which is 64-bit, or something else, which is 32-bit.
uname -a
_64 will mean that its 64 bit. otherwise its 32 bit