you need to look.at real world benchmarks based on.the games you.want to play and see what real world differences they play
just asking.about badwith isnt enough you need.to ask yourself what games with what config you want to play as.well.as get a budget together to determine which card is best for your situation
If you want to get really technical, you first need to know your motherboard supports it's expansion type which i assume is PCIe x16, which all newer computers do have. You also need to make sure the card will fit in your case. If you have a typical mid tower (about 14 inches tall) it should fit. Anything smaller and you should research it. From there you need to know that your power supply in your computer has enough wattage to supply the required power to the video card and rest of the computer (the average video card requires more power than every other component combined), and that the power supply has the right connector available for your video card (assuming your newer model video card will need it). After that or honestly before all that you just need to know which type of applications (games/software/programs) you are going to run to determine how demanding they are so you know how good of a card to get. As the other suggestions say, running the tests is an excellent way of achieving this if you don't already know from experience.