I have a Dell Inspiron 580 (i580-6654NBC) core i3 550
I want to upgrade the graphics card that I already have to a evga gtx 780 6gb that I found for a great price
Problem is I don't know if it's compatible or if it will even fit in the case
You would have to upgrade to a better power supply. Depending on the cost of the GTX 780 (+ cost of new PSU) you might be better off with a new GTX 1050 Ti and you won't have to upgrade your PSU.
Yeah well I'm considering just scraping it and getting a new pc eventually, don't want to waste the time building it up for it only to run older games on low to medium settings