Hi Chris,
Whether or not your Mac will have the 65-bit support for it is kind of a hidden feature since they never published an official list of what has the 64-bit version and what not. The y started to ship the Macs that support the 64-bit versions of Boot Camp Vista in very late 2007. So if your Mac was manufactured and shipped in late 2007 it would support this - then again it would probably have been shipped with Leopard preinstalled with it.
The earlier versions of Macs do support 64-bit ... but not in Boot Camp
so if your Mac was manufactured earlier, but is still a Core 2 Duo, then the 64-bit Windows OS may be accessible only with VMware Fusion - maybe with latest Parallels too if they updated it in the past few months.
If you are not sure if your Mac has that feature, when did you buy it? Or which model and proc. speed does it have, and what are the first 5 digits of the serial number? (this may help to narrow it down in case of doubt)