No, you would only see visible speedup in intensive tasks. Ability to use more RAM and wider registers are the main advantages.
The amd x86_64 arch is clearly going faster through the use of more and extended registers when using the 64 bit mode. Conversely the intel core 2 architecture is known (That's what I was told at least) to be a touch slower in 64 bit than in 32 bit mode.
If I don't mistake the problem of core2 is due to the fact that the so called macrofusion (you know this things that allows core2 processors to decode 2 x86 instructions at the same time) is not available in 64 bits mode.
Notice that there is no such problem with the Nehalem or barcelona architectures. Also, I don't feel my core2 E6750 to be that slow under FC10 (64bit). I have no benchmark results to show but I don't expect the performances to drop by --- lets take a large margin --- 10 %. So you can confidently erase your old windows 32 bits ; try the 64 bits version ; face a mountain of bugs and other problems ; and then switch to any reliable POSIX compliant OS :-).
How come its not enabled by default in vista 32bit? Would stop alot of people complaining...
It would make a lot of people complain : PAE can be efficient for some kind of work but it can be very slow and inefficient. So unless you're a computer expert you don't want this unable. And if you're such an expert working with windows one expects you to be able to configure your OS according to your needs.