Most apps run at least as fast as pn 32bit (under Linux, that is). Some apps use 64bit integers, like ogg/vorbis and some others, those apps will see a nice speedup...
Linux x86_64 is about 30% faster than 32bit on the same CPU, some applications see no increase, for others it's up to 350% faster (e.g. oggenc)...
"Computer games don't affect kids; I mean if Pac-Man affected us as kids, we'd all be running around in darkened rooms, munching magic pills and listening to repetitive electronic music."
-- Kristian Wilson, Nintendo, Inc, 1989.