That's 3 questions...but luckly I have 3 answers.
1) There is no one site to get the best price, although you won't lose too much if you buy the stuff from a Microcenter. the "best price" is discounts offered by the seller. Although Newegg and Tigerdirect have prices that are "regular" that doesn't change often unless there are rebates or discounts
2) Yes! Normally you need to choose between 64 bit or 32 bit when buying OEM while retail normally has both
3) If you don't know exactly how to build a computer you can watch this 3 series video made by NewEgg:part 1Part 2part 3