GTX 750 PSU

Real 400W even can handle R9 290 so all about efficiency of PSU.

The GTX 750 uses about 55w if you check:
Game-debate GTX750
After some searching around I've seen the GTX750 don't get above the 90w.
So assuming you don't have an power hungry CPU you'll be fine.
Of course age and quality of Power supply unit is important as well!

I think 300W is needed to properly let the GPU work, and 400W to be sure it gets enough power.

If you want to overclock, or want to have some room for upgrading in the future you can always get something with a little more wattage; after all the PSU only consumes what it needs

Also if you are concerned about cost you can get those 80+ Gold or platinum ones that are more efficient (less energy wasted so you save on electricity bills)

You need at least 1000w to run 2 750's in SLI overclocked.

It draws about 60W at stock. Shouldn't go over 75W since it draws all its power from the motherboard.

Assuming the rest of your system draws around 150W, a decent 300W would be enough to power it. Again, this is based on how much the rest of your system needs and how much your PSU can output on the +12V rail.

well i have gtx 660 it says minimum 450W but for real it needs 630W minimum to run, i mean pc was working fine , but when i launched the game , my pc was shutting down, so i was unable to play any game until i replaced my psu for 750 w now it works fine))

My 280W PSU drives:

i7 - 3770, 16GB ram, 2TB drive, GT 640

And considering this
http://www.hwcompare.com/17304/geforce-gt-640-ddr3-vs-geforce-gtx-750-ti/

my guess is you can do with a 300W PSU

Depending on how old your PSU is, it's always a good choice to get a new and better one if it's been running for about 4-5 years. I think you will do fine with a 400 one if so. The PSU tend to lose its power after 4-5 years. So to be on the safe side and to make sure your PC will get enough power to the other parts, get a 400.

I'm currently using a 550 PSU for my PC, with a brand new 770 card (got it 2 days ago), then again I have a few other pieces that require power. I used to have a 470 card which was a beast when it came to power consumption, it was horrible to be honest. I don't think you'll have that same problem with yours, since the 470 was known for high electricity bills, hehe.

If you have a not extremely bad PSU, you can use the 750 withouth problem

Generic answser: look at the specifications given by the manufacturer of the card (not "Nvidia GTX 780" but, for example, Gigabyte model GV-N780GHZ-3GD). The useful info is something like that: 600 W with a minimum of 12v constant rating of 42 A.
After that, you have to check the power supply unit specifications for the max amps available (12v). For another example, a Corsair CMPSU-600CX gives 40 A max.
If PSU value > card value, you're good to go. With the examples I gave, the PSU would be too weak for the card. Please note that this is a quick and generic answer, a more precise one should take into account you other peripherals (HDD and so on).