It seems the MoBO determines alot of what components are used if you were building a computer. What determines whether you use 32bit os or 64 bit os in that computer. Are they interchangeable? Lets say would my old test computer support 64 bit os if it were originally a 32bit os. Not something I am thinking of doing but just want to know what tells the computer which one it has to use. Hopes this makes sense.

Most cpus and motherboard will support 64bit, problem is its a different disc you use to install it (unless you buy the more expensive retail version, and most computers are oem so you wouldnt be able to use the disc to install on someone elses computer even if its the same operating system), the difference in performance will be small if any, unless your into gameing wouldnt bother stick to 32bit.