As far as for your GPU. You still didnt state what PSU and case you have. So theres no way to know if any GPU you buy will have the right connectors for a new GPU, if it can safely provide enough power, or even if it will physically fit inside your case.Minimum specs: it's pretty likely that you'll be able to play the whole game at the lowest settings. If you haven't got this level of hardware, they don't even want you to try to play the game. Let me emphasize: a company does not want your money! Let that sink in before you buy. Sure, some people will be able to squeak through with even lower stats, but when the people who stand to make a profit off of that are telling you that you shouldn't try, maybe you should go play something else.
Everything else is bull. If you've got the minimum, you can (barely) play the game. If you've got better, the game will do better. If you want to know how various hardware advances help various systems within the game, you'll need to look elsewhere for a detailed analysis.