In my experience, I always thought I might need to upgrade something -anything- on one of my rigs, but found that I ended up building a whole new contraption anyway. So my advice is: Build whatever is in budget. Really. Come up with a budget, and then fit the pieces in. Spending or saving an extra $50 seems like an issue now, but three years from now when you're playing your machine you'll either wish you spent more or not even notice it. I'm frugal so I salvage the innards for my machines from friends, craigslist, leftovers from my own upgrades, etc. Why buy brand new parts that are at best going to get minimal usage anyway? Sure, you think you'll play it a lot. But you won't. You'll play it in spurts - an hour here, three hours there - maybe a total of 150 hours a year.
What's your favorite game? That's how I started. I made sure that whatever I built, I better damn sure be able to play my favorite game(s). Forget about everything else and start there.