Here's a tool to help you pick an nVidia graphics card.
You'll need to know your budget, the amount of space available in your box, and the wattage of your power supply. To check those, you'll have to open your computer up, measure the space from the port of your current graphics card straight back, and look for a sticker on your power supply that labels the wattage.
Since you want to play upcoming games, I wouldn't recommend anything less than the GeForce GTS 450. If you want to be on the higher end of the spectrum, the GeForce 560 Ti is the best product for the money right now - not as expensive as the very newest model, but a highly-rated and high-performing card.