you need to know how you will use it. Are you a gamer? Into digital photography? Music? Do you plan on learning new skills? Your current and future needs will determine what hardware you'll want included.
I haven't reinstalled Windows XP as of yet. My computers seem to be running fine and I don't feel the need. I stopped doing much tweaking years ago after making such a mess of a computer, I had to start over. I find when I buy a powerful computer from the start, I don't need to do much tweaking.
Is there like a checklist of things you need to do to optimize windows when buying a new computer or installing or reinstalling windows? Like what programs you might need, which one's to delete, etc. What system tweaks to make. I thought I saw one a long time ago but I tried searching google and didn't come up with anything.