I am not computer science person so I do not know the exact reasons why people think that 64 bit operating systems perform better than 32 bit.
What I understand that might be 64 bit operating systems is futuristic choice. At least on today's date one has to struggle a lot while using 64 bit operating system.
Some problems I have faced -
1. In greediness to have support for 4GB RAM (it was my misconception - actually support 32bit version can make use of 4GB RAM), I purchased 64 bit - Vista home premium (waste of money) with Dell Studio 15. It was showing video RAM as 256 MB when actually it was 512 MB. After about 30-40 mins discussion with Dell service person, he concluded that this is problem with 64 bit Vista.
2. For Ubuntu 9.04 32 bit Operating system, I got sound and video card drivers very quickly and it worked fine. When I tried Ubuntu 9.04 64bit edition I could not get video card drivers (I am not geek to write own video card drivers, or might be I should have tried more).
3. I am currently facing same sound and video card drivers problem with Fedora 11 64 bit operating system as well. On internet I could see support for 32 bit Fedora 11 for ATI Radeon graphics card but not for 64 bit (source code was not available as far as I remember). I am quite sure that sound problem must be easy to solve.
In addition to this all above problems - I have not seen any noticeable performance improvement in 64 bit operating systems.
I do not have any doubt that future belongs to 64 bit operating systems. But at least on today's date 32 bit Operating System is better option in my opinion.
Like many people even I had misconception that RAM higher than 4GB is not supported by 32 bit operating system. Follwoing article (again thanks to Yogeshwar) might be of help -
http://en.wikipedia.org/wiki/Physical_Address_Extension
I must mention few things before finishing this post
1. I am only talking (writing :)) here about operating systems. Please do not misinterpret with 64 bit architecture. If you want to buy new PC - better go for 64 bit hardware (but as per my experience 32 bit operating system)
2. Probably support for NVIDIA graphic cards is available for most of 64 bit Linux distros.
It is useless to have 64 bit architecture and multi-core processors unless the developers writing applications for these hardware make use of their capabilities.
For some people running 32 bit operating system on 64 bit architecture might look like running bullock cart on national highway, my problem is though the option of car is available I do not have guarantee that I will get fuel stations and shop to repair puncher on highway. So I will better drive car when these services are guaranteed.