32bit or 64bit PHP on 64bit capable processor/64-bit Windows Server 2008/IIS7

I plan to install PHP on a system running on AMD Opteron/Windows Server 2008/IIS7. According to the device manager the OS is a 64-bit Operating System. So which flavor of PHP should I install on it. If 64-bit PHP 5.3.x then will there be any gotchas? Or should I expect 32-bit PHP to run smoothly without any (performance) problems?

PS: in case you're wondering -- WPI sucked and did not work at all. It tried to download 32-bit/NTS version of PHP for Windows and failed repeatedly.

You should use 64bit PHP for significant performance improvement, though you can try the 32bit too !