To install the latest version of OS X - Snow Leopard - you need to make sure you have a 64-bit processor and a 64 bit EFI.
Of course, this is not something many people consider when they buy a Mac and if you're wondering what your system is, Startup Mode Selector could help. Startup Mode Selector is a small application which reveals all this and more including whether the MacOS X Kernel set to boot is running in 32-bit or in 64-bit mode (64-bit being essential to run Snow Leopard of course).
Startup Mode Selector also allows you to select whether you want to start the MacOS X Kernel in 32-bit or in 64-bit mode although the developers warn that this setting might be ignored on MacBooks, depending on how Apple implements the restriction. If you have both, it allows you to startup in either 32 bit or 64 bit mode. The application also includes some handy information explaining just what the hell all the fuss is over 32-bit and 64-bit Macs. The technicalities of all this make very heavy reading for the average user but you don't really need to understand all that - just use Startup Mode Selector to see exactly how many bits you have.
Startup Mode Selector doesn't do anything that you can't already do in Terminal but it makes it much easier for the Terminal shy and will help you see once and for whether you are on a 32 or 64 bit Mac.