|"I was wondering if EM64T adds any significant speed when using a 32-bit OS."|
Nope. The feature isn't even used by 32-bit OSs.
Windows x64 isn't much faster than Win x86. A few years ago I tried both versions of the Vista Beta on my 3.2GHz Opteron X2 machine. Vista 64 was like 3% faster in CPU benchmarks than Vista 32.
Although, I now run 64-bit Photoshop CS5 on Win7 and it does seem quite a bit faster than the 32-bit CS3 I was using a year ago (same machine, same OS). I'm not sure if the speed comes from the program's 64-bitness or something else. :)
Super PIII | Unlocked ES Tualatin @ 1.8GHz (150x12, 1.65v, 512K L2),
2GB PC2700 | 500GB | Radeon x1950Pro | Apollo Pro 266T | Win7 Pro