Installing Operating system with 64 bit and 32 bit

On a 32-bit operating system, you are restricted to a maximum of 4 gigabytes of RAM. On a 64-bit operating system, you really do not have a limit.
There are many more general-purpose CPU registers in 64-bit mode. Registers are the fastest memory in your entire system. There are only 8 in 32-bit mode and 16 general purpose registers in 64-bit mode. In scientific computing applications I've written, I've seen up to a 30% performance boost by recompiling in 64-bit mode (my application could really use the extra registers).
Some of the things said in this thread (like the doubling of # registers) only apply to x86-> x86_64, not to 64-bit in general. Just like the fact that under x86_64 one guaranteed has SSE2, 686 opcodes and a cheap way to do PIC. These features are strictly not about 64-bit, but about cutting legacy and remedying known x86 limitations