Apple put up a page in the knowledge base explaining how to find out if your Mac is 32 or 64 bits enabled. It will explain how to find out which CPU you have and in that lies the answer.

The short answer is:

Intel Core Solo is 32 bit
Intel Core Duo is 32 bit
Intel Core 2 Duo is 64 bit
Intel Quad-Core Xeon is 64 bit

Read more about it here.