32bit cannot use all of the power that most CPU's can use. So 64bit was introduced, depending on the spec's of your PC it will either have a major advantage or none at all.
As for windows systems and memory allotment's,
Win 7
Basic 8gb, good for most games.
Pro 16gb, Even better for games.
Ultimate 196gb. < Most PC's useless above 16gb unless you do video rendering, or your motherboard, cpu, gpu are all geared for playing games at max performance.