32bit Vs 64bit OS

[Alan Cox]

O> Agreed. First, the 64-bit OS itself should be a bit faster than a 32-bit 
> OS on the same platform. But, in my experience with 64-bit going back 16 
> years there are some applications that will run faster if built for 
> 32-bit and some will run faster if built for 64-bit. There are some 
> tradeoffs. The sizes of libraries and applications will be larger in 
> 64-bit. You will need both 64-bit and 32-bit libraries if you want to 
> run 32-bit apps.

The x86 case is a bit different to the others (indeed for example sparc64
user space is mostly faster 32bit)

On x86 the CPU has a real shortage of registers. The 64bit mode changes
this somewhat which means its not a simple 64bit extend but also other
performance improving features that become available.

Alan