I'll be looking to overclock the i5 to 4.5ghz, and I may add another 16GB ram or an additional 980Ti within the next few years. Not for practicality. For the e-peen.
Pretty sure that you can list all your parts that will be in your build out and it will give approx wattage use.
From there I think you would want to target a PSU that is double what that wattage is.
If I look at this build I'm working on: http://pcpartpicker.com/p/hBZcWZ
I can see it uses around 342w max... you don't want to run your PSU at 80% capacity constantly, as it will get quite hot (and most likely die early) So I'm shooting for a PSU around 700. I could go as low as 600 most likely but I'm thinking I'd rather error with a slightly stronger one that weaker one.
I would shoot for around 2x the approx max wattage.
If you want to add that additional card, make sure you temporarily add a second card to your build out (or just double the wattage used for that single card)
Don't forget other components like your SSD that you aren't buying per se, but will add to your build out.