Hello and welcome to the forum.
1. It depends on how much RAM you have installed.
2. (quoted from Microsoft)
"The terms 32-bit and 64-bit refer to the way a computer's processor (also called a CPU), handles information. The 64-bit version of Windows handles large amounts of random access memory (RAM) more effectively than a 32-bit system."
And i don't have an answer for your last question, maybe someone can fill in where i missed. Sorry.