To be honest, I'm not a person who could recommend you a graphic card to buy. When my old GPU on my newer desktop PC died, I upgraded it with NVIDIA GeForce GT 730, which was actually one of the cheapest GPU's I could find with 2GB of memory, and it works great for me.
But I play most of my games in 1024x768, or some lower widescreen resolutions. For example, I am able to play Stronghold Crusader 2 in resolution 1280x800 and medium or low graphic settings, and I play Stronghold 2 in 1024x768 resolution and high graphic settings. I'm just not demanding when it comes to graphics, I'm most satisfied when the game runs smoothly. Well, sure, I would be happy if I was able to play Stronghold Crusader 2 in highest graphic settings, but with my incomes I'm not ready to spend that much money for that pleasure.
Considering your configuration, I assume it's an on older PC, and I fear that upgrading it wouldn't be a good option for you. Especially if you were thinking about buying new parts, because those new parts might not be compatible with your older motherboard. You might be able to improve your experience by upgrading with used parts, but I fear not as much as you hoped for.
Oh, but what version of Stronghold 2 do you have? Is it the Steam version, or a retail copy?
Anyway, if you want to have better experience, I think your best chances would be buying a new PC which would be easily upgradable later if needed. (You might ask somebody to help you with the choice of your new PC if you don't know much about matter.) If you are on low budget, then either be patient and do the first thing, or buy a used PC for cheap price from somebody. Again, with help from somebody who knows more than you about this topic.
But if you don't have experience with buying and upgrading PC's, I think you should be patient, and buy a new one in a reliable store. Not in some store in a basement. This way you have your warranty, and you're safe that you're getting what you wanted, and that nobody has tricked you.
Sorry for long reply, but I don't know how old are you, nor in which country you live, but from your questions I remembered how I wanted to upgrade my PC when I was a kid, and how I had painful experience with bad people who only wanted to trick me. And how my father needed to step in. So, if you want to upgrade your PC with used parts, do it with somebody who works with computer, and who you trust! If you want to buy new PC, do it in a reliable store who offer you official warranty for stuff they sell, no matter if it is slightly more expensive.