What buying a more powerful graohic cards do is that it will allow the game to run at a higher internal resolution, as well as being able to increase the anisotropic filtering and anti aliasingand yes, it will increase your fps. I don't know much about your CPU but I do know that AMD doesn't really perform well with Dolphin and many AMD CPU's struggle to keep up with Intel CPU's so you might not be able to play some more demanding games.. the 3.0 build is very VERY old by now and surely full of glitches and bugs, actually the 4.0.2xxx builds now run much faster and have less bugs so you might want to download the latest build
The right man in the wrong place can make all the difference in the world.