For modern games your looking at low settings with about 20 fps..

Honestly, you should probably save up until you can buy a better card. I once bought a GTX 650 because of the reviews on Amazon, but it wasn't as good as they made it seem to be.

If you want to get a better card, but can't wait, then you can check out this guy [ Register or Signin to view external links. ]

He has good reviews, sells at amazing prices, and seems reliable. He has a 270x for around $110, which will run almost every game at high-ultra settings. Even though some of the cards were used for cryptocurrency mining, most people got many moths of performance, and then the only thing they needed to replace to get the card working again was the fan.

[ Register or Signin to view external links. ] is a post about him on reddit.

If new cards are more your type, then you should save until you can get a 260x. It's definitely better than a 640.
[ Register or Signin to view external links. ] is a comparison between the 640 and the 260x.