For just about every normal game and application, I haven't been able to feel a speed hit when running 32bit applications under a 64bit OS (this is perceived speed, I haven't bothered to run anything resembling a scientific benchmark) - and things in general
has felt a bit smoother running 64bit.
There's been a single instance where performance took an abysmal hit, though: Foxit PDF reader. I'm not sure what
the problem was, but at least on XP64, rendering complex PDFs is (or was) extremely
much slower when running 32bit Foxit on 64bit Windows - so slower that running 32bit foxit in a 32bit XP virtual machine in vmware on the 64bit host actually rendered those PDFs faster
I haven't dealt with complex PDFs in a while though, so I don't know if the problem has been fixed with later Foxit versions, and I'm also running Win7-64 right now, where the situation might be different. But it's been one
application with slowdown, and pretty special circumstances.