I'd say skip i7, just get i5; the real i7 benefits are not _that_ big a deal relative to cost.. but that said, if you can get i7 for next to nothing over its i5 equivilent, nab it.
i5 + a good video card and you're in the cleart.
But if you want to run retro games, old arcvade games to display on a project or onto real arcade hardware, or other strange things, you need to be very specific about your requirements.
But if you just want to run Skyrim or the like, then you can get any 'pretty good' video card and be in the clear; the question then becomes.. do you want a crazy awesome current display (SLI 2 video cards, say), or future proofing (single really buff video card)... but video cards rfamp up in price quite heavily once you get past the current 'pretty good' cards.
So its, as in all things, a question of what you want and what your budget is
For case and power supply and such, you can go pretty inexpensive.. cost is for looks; if you don't mind ugly, some cheap coolermaster will have a big watt power supply thats not half bad, and a big airy ugly as sin case. If you want a good looking case it'll cost, or a small noisey or quiet case.. cost.
jeff