Upgraded from a HD 6670, I just purchased a GT 640 today on sale for $85...
I did a little bit of overclocking:
Clock: 384 cores @ 1,000 MHz (900 MHz Standard)
Memory: 1GB DDR3 @ 2,400MHz (1782 MHz Standard)
So far so good, for a DDR3 card, it seems to be holding it's own... I can play all my games at full 1920 x 1080, with medium to high settings in most. I'd say its probably on par with the HD 6750. Which may not mean much when the 640 is suppose to compete with the 7750... But remember it's running off of DDR3 Memory, not GDDR5 like in the 6750/7750. So Nvidia must be doing something right to get that kind of performance out of DDR3 memory.
I just wish that one day we'll see a GDDR5 GT640, cus it'd just blow everything out of the water!