Due to software limitations, that register width is usually a choice of 8, 16, 32 or 64 (no intermediate values). So a 32-bit OS is an OS written to run on a CPU with 32-bit wide registers, a 64-bit OS is written to run on a CPU with 64-bit wide registers.
I have to buy a laptop but my laptop's RAM only 2GB, i think its not enough memory RAM on my laptop. I have add external 2GB RAM and total RAM size is a 4RAM, so i have increased by RAM size because i give some high speed in my laptop and i think it is a better for my laptop.