Depends what settings you're happy with. If medium is fine, you'll do good with a single 780/290. If you want high settings, you'll need a 780 ti. For maxing games out with 120fps you'll need SLI 780 or Crossfire 290
Eh, I mean getting more than 60 fps. (enough to notice a difference over 60hz). If he turned down some settings he'd be able to get 144fps with the cards I first mentioned.
Also depends on the game you want to run at 144Hz i assume its a FPS... What is your CPU (might not matter what video card you get if your CPU is low end)?
Also, there are games that have a locked frame rate anyways. Just so that's out there.
As for the 144Hz monitor, there's not much more than a 60Hz monitor like Alex says. Main difference in my opinion is just that it helps lessen the amount of screen tearing since it can run at a higher frequency.