While others may have different opinions, I would find a PSU calculator and put in the info of your system now. it will tell you what size PSU you need now, or put another way, how much power your system is currently pulling. When looking at video cards, they will usually tell you how much power they pull and some even tell you the minimum PSU requirements. Next, after deciding on the new video card and ram, go back to the power calculator and add those components and see if they fall under
the size of your current PSU.
Just for your info. I have a Dell 580s. I wrestled with some of the same problems you are facing now. That is the reason I have the system I am running. I considered most of the suggestions you have received. New ram, decent video card, PSU requirements, and even a new CPU. I considered those costs. That is when I realized that for just a few more dollars I could have a new and better system than if I had made those upgrades to the Dell. I have no idea of your financial situation or the costs of components in your country, but You may want to look at the situation in that way.
There is a power calculator here.
This site is in the USA, so prices will not be the same, but while on this site you can look at some video cards and the details. When you find one you are interested in, just check prices in your country. Sorry, but I am not familar with the sites in your country so I provided you with what I an familar with.