Finding the right parts for your computer is one thing, but what about selecting an operating system? This could be a several hundred dollar investment, or it can be completely free. If you're looking to save money, you may want to give serious thought to Linux. On the other hand, you might want to stick with what you're familiar with - Windows, in it's various flavors. For help choosing, I suggest reading: Choosing an Operating System for Your New Build.
While you're doing some reading there, educate yourself about what is possible by switching to Linux in: What are the Advantages of Linux? Since there are so many hundreds of Linux Distributions you may be wondering which one to choose. Find the answer in: Which is the Best Linux Distribution for a New User? For more Linux Distribution information, see the Linux Distribution List - By Type.