Everything works well, but now I have to change the Video Card and get a GTX 550 Ti. My problem is that I've seen in different sites the power consumption of the GTX 550 Ti, and it varies from 116w to 400w, so I don't really know if it will work on my generic PSU.
It's not that I don't want to buy a new PSU, it's just that there is no "Thermaltake", "CoolerMaster", etc. in my country, just random brands that are not listed as good PSUs. For example, a guy offered me a "RAMBO" PSU (I know, it must be generic) of 1000W for U$100. The only TWO non-generic PSUs cost around U$300 (800w) and U$400 (1200w), which is extremely out of my price range.
So. I want to know if I can run this system with an ASUS GTX 550 Ti instead of the XFX GT 520, considering that my PSU is a generic Delux ATX-750W P4 with 51A.
That's horrible luck that you can't really get any better PSU, but the one you have should work. Even the worst 750W units should give you around 500W I would imagine, which is still more than enough for that system. I'd say you should be fine.