AMD cards are probably a bad example since they are decent at Ethash but horrible at Keccak…
For fun, I am mining on a 1GB Radeon HD 7750 here and while not really getting a lot out of it, it is certainly possible and gives me a hashrate of around 70MH/s. This only makes sense because the card draws a meager 40W. The GTX 640 will probably be around the same neighborhood. These cards, while operable, are really too weak to make any serious dents.
Are you sure you set up the card correctly? I am running a GTX 970 and I’m getting twice that hashrate.
Make sure to enable GPU processing in the card’s setting using nVidia’s software.