It shouldn't give you any advantage performance-wise for individual apps, as the OS can run 64-bit apps even if the kernel is still running in 32-bit mode (which is the only thing this setting controls). The reason to keep the kernel running in 32-bit mode is to allow 3rd party "kernel extensions" that are written in 32 bit mode to still work.
Mac OS X is built so that almost the entire operating system and applications can happily run in 64 bit mode even if the kernel itself is still in 32 bits. Unless you are a developer of kernel extensions, I think you should keep your system running in 32 bit mode.
FYI, the 'kernel' is a small (but very important) piece of the operating system. It acts like its own application once your system is started up, and basically takes care of low level OS-related issues. When you are running a 64-bit app, you are completely in "64 bit mode" from the application's point of view, and have all the advantages of that mode. "64 bits" vs "32 bits" basically controls an applications memory space, and how much memory it can actually use. That's why 32 bit applications can only use up to 4 GB of RAM. That's the max number you can have using 32 bits.
