Understanding 32 and 64-bit options

I am a photographer, and understand that operating in 64 bit mode is an advantage. I know that I have the 64 bit capability with my Intel Core 2 Duo iMac (currently running OS 10.6.8, soon to upgrade to Lion), but I'm not certain if I actually am running in 64-bit mode, or if it is possible to have it running in 32-bit mode. If there is an option, how do I find out what I am actually running, and how do I control it?

Thanks for your help!