Q: Do I need Snow Leopard to run 64-bit applications?
A: No. Leopard also has support for 64-bit applications. Even Tiger and Panther have limited support for 64-bit applications. The main limiting factor on older operating systems was the less-prevalent existence of 64-bit CPUs and machine limits below 4GB of physical memory.
"Tiger is also interesting in that it will actually support full 64-bit processing on the PowerPC processors that Apple has been shipping for years. Both the prior "Smeagol" 10.2.7 and "Panther" 10.3 releases of Mac OS X ran in a hybrid 32-bit/64-bit mode. " - The Unix Guardian.Q: Does my EFI need to be 64-bit to run 64-bit applications?
"Mac OS X Tiger delivers the power of 64-bit computing to your Mac. Build and run a new generation of 64-bit applications that address massive amounts of memory, without compromising the performance of your existing 32-bit applications." - Apple.
A: No. EFI does not have any affect on applications running in 64-bit mode.
Q: Do I need to load the 64-bit Kernel & Extensions to run 64-bit applications?
A: No. 64-bit Kernel & Extensions will only make a difference on machines with 32GB of memory or more.
"In fact, the only big advantage of booting into a 64-bit kernel would be the ability to use more than 32 gigabytes of RAM. There aren't any Macs that can do that now, anyway, due to hardware limitations." - MacWorldQ: I've read I can't run 64-bit Applications in Snow Leopard unless I run 64-bit Kernel & Extensions, is this true?
A: Again no. You will be able to run all of the included Apple 64-bit applications and 3rd-party 64-bit applications on Snow Leopard without a 64-bit kernel or extensions.
"So, bottom line: If you’ve got a Core 2 Duo or Xeon based Mac — any Intel Mac not running a Core Duo or Core Solo processor — you’ll be able to run applications in 64-bit mode, which will in turn be able to take advantage of faster 64-bit registers and math routines as well as access massive amounts of memory." - MacWorld
Q: Without the 64-bit Kernel & Extensions, will my 64-bit apps still be able to use more then 4GB or RAM?
"Applications running in Snow Leopard will have access to a full 16 exabyte virtual address space, just the same as if they were running in a 64-bit kernel." - MacWorldQ: Why is there so much new talk about 64-bit with Snow Leopard if 64-bit has been supported for so long?
A: Snow Leopard increases Apple's commitment to 64-bit by introducing 64-bit versions of most of its popular Applications and many services in 10.6. This means Apple has re-written much of Leopard to take advantage a 64-bit environment and more then 4GB of physical memory.
"Nearly all system applications — including the Finder, Mail, Safari, iCal, and iChat — are now built with 64-bit code. So not only are they able to take full advantage of all the memory in your Mac, but the move to 64-bit applications also boosts overall performance." - Apple