I am using 64 bit, so I think Vista is becoming a better choice. I have found it to be harder to find drivers for xp64. Getting rid of all the crap that is preinstalled on new laptops increases performance way more than the operating system. My laptop was painfully slow until I reformatted it.
When looking at laptops right now, it is easy to tell if the os is 32bit or 64 bit. 32 bit can't handle more than 3 gb of ram, so the manufacturers are only offering 3. If it comes with 4gb, it is most likely 64 bit.