Should I buy one part at a time starting with the case or know what I want and save my money and buy it all at once?
All at once. If you buy it over time, it's going to be a project that never gets done.
I'm looking to spend around 150 for a case, Im more concerned for the ventilation over looks but looks are nice.
SilverStone focuses a lot on ventilation, but they're a boutique brand. If you can afford one, I'd get it. Otherwise other options to look at are NZXT, Corsair, Cooler Master and Antec. I think Antec is also a little more focused on ventilation like SilverStone, but they don't charge up the rear. The other's I'm sort of flaky on, but they're usually the most bought brands.
What video card should I get, if it's pretty expensive but really worth it I'm willing to save up for it.
If it's now, a GeForce GTX 670. But considering you probably don't have enough money for everything yet, the future could hold something nicer. But this alone can play BF3 on Ultra rather smoothly.
Save up about $1000-$1200, you'll have your rig soon enough.