Just in terms of speed, 64bit won't be slower than a 32bit if your computer supports it, right?
My sister got a HP from Futureshop in Jan., and we were surprised that it was a 64bit Vista. But everything works on it, or at least the software that we use. So I'm not worried about software or hardware issue.
Rather, I heard (yea, you can't believe everything you hear), that although 64bit is faster, it uses more RAM in general (ie even during idle)

. That's why I'm just making sure that installing 64bit can't make your computer slower than it was on 32bit.