So yeah, Snow Leopard boots in 32-bit mode by default, but that’s not a big problem as it only affects the kernel. 64-bit applications (like Finder, Mail, Safari, iCal, and iChat) will run just fine (including benefits) on a 32-bit kernel in Mac OS X!
One can trigger OSX into booting the 64-bit kernel by either pressing the 6 and 4 key during boot, or by using a tool such as Startup Mode Selector.
The only consequence when booting a 64-bit kernel, is that the kernel will only load 64-bit kernel extensions (kexts). So what’s the problem then? Well, applications such as VMWare Fusion currently still rely on 32-bit kexts, making them unusable with a 64-bit kernel.
In short: for now, just stick with the 32-bit kernel as you’ll still see the benefits. When apps that still use 32-bit kexts have a new 64-bit release, then you can/should go for a 64-bit kernel 😉