DanFauOrigami :
It says it cannot exeed 20A
And that is for the 12V1 & 12V2 rails only ?
If yes, it looks like it should work then, 20A = 20 amps (20*12 =240W). So it's saying those rails combined can deliver up to 20 amps but no more than that. As long as your system is minimal i.e 1x HDD 1x DVD etc... it should be fine.
if it doesn't work, then you'd need to replace it. But you don't need anything expensive. EVGA or Corsair 430W is fine. And getting a more beefy GPU will just be increasing the likelihood or amount of a bottleneck.
Edit: Also ensure it's NOT a 750ti that needs a 6-pin PCI-E connector either, as those use more power. You need to use as little power requirement as possible.