The second card will provide you a marginal-at-best improvement in performance. It would be like trading a Ninja 250 for a CBR250.
At $65, a Radeon 6670
would give you acceptable performance at 1080p with medium graphics. However, with improved graphics or the next great game that comes along, you may find yourself sorely disappointed. Plain old Chevrolet quality, but it gets the job done. Note that the first card, the GT 640, is on par with the 6670, but the 6670 should give about 15% better performance.
Moving up to $130, the Radeon 7770
will give you good performance on 1080p resolution at any quality setting. Unlike the previous card, you can expect this device to be usable for at least 5 years as the gaming industry moves forward. Consider this the Buick of graphics cards.
I use this resource to stay up to date on desktop graphics:
If you want to know the comparable level of performance between two products, reference this chart:
Edit: Kinda nice how the GTX650
@ $115 fits right between those two options.
Edit 2: Fixed an error that Dave highlights below.