How can I boot an Intel Core i7 iMac in 64-bit mode?
To start up with a special kernel check this from Apple: Starting up with the 32-bit or 64-bit kernel in 10.6 and later:


  Startup key combination (for current startup only)
  
  If your Mac uses the 32-bit kernel by default, but supports the 64-bit
  kernel, you can start up using the 64-bit kernel by holding the
  6 and 4 keys during startup.
  
  If your Mac uses the 64-bit kernel by default, you can start up with
  the 32-bit kernel by holding the 3 and 2 keys
  during startup.
  
  Your Mac will revert to the default kernel the next time you reboot
  it.
  
  On-disk setting (persistent)
  
  To select the 64-bit kernel for the current startup disk, use the
  following command in Terminal:
  
  sudo systemsetup -setkernelbootarchitecture x86_64
  
  To select the 32-bit kernel for the current startup disk, use the
  following command in Terminal:
  
  sudo systemsetup -setkernelbootarchitecture i386
  
  Note: This setting is stored in the
  /Library/Preferences/SystemConfiguration/com.apple.Boot.plist file
  and will take effect every time you start up from this disk. If you
  start up from a different disk, the setting on that disk, or the
  hardware default, will take effect.


To determine your currently running kernel open Terminal.app and enter:
uname -a
Running with the 64-bit kernel, the output will end with RELEASE_X86_64 x86_64. Running with the 32-bit kernel, the output will end with RELEASE_I386 i386.

In your case you just have to update to Mavericks (Mac OS X 10.9). Mavericks doesn't contain a 32-bit kernel anymore and since your i7-processor is 64-bit, everything just works right out of the box without key strokes or modifying com.apple.Boot.plist.