There's no "emulation" here (unless we're talking about Itanium, not x64).
Basically, the CPU is just switched into 32bit mode (when executing the 32bit code). The overhead is so small that it's kinda pointless to talk about it.
I do not know where you get your information from. All 32 bit applications run in Vista/7 64 bit are being run in a x86 emulator.
Sure, you can call the WoW64 subsystem an "emulator", if you wish - cause it somehow "emulates" the 32bit Windows environment.
However, there is no instruction emulation there - once the thunking (switch into the 32bit/64bit mode is made - with the corresponding parameter translation, if calling Win32 API), the code runs natively.
It would be rather stupid to emulate the 32bit code if the CPU can run it natively, wouldn't it?