There are better cards for the same price, such as the 7750/70.
As one example of many: http://us.ncix.com/products/?sku=72479
In the original review of the 640, it wasn't as good as the 7750. In the recent Crysis 3 review, the 7750 performed on par with the 650 (The 640 was not considered).
If you insist on Nvidia, get the 650/650ti: http://us.ncix.com/products/?sku=76612
Personally, I'd get the 7770 for ~$100, or go up to something like the 7850 / 660.
Just my 2 cents.