1GB GDDR5 NVIDIA® GeForce® GTX 555 vs NVIDIA® GeForce® GT 640 1GB GDDR5

one thing there is nothing like gtx 640 its only gt640 & which is not ddr5 so Gtx 555 takes the lead gtx 555 is having 192 bit memory as well as sli connector with ddr5

The GTX 555 is a tiny bit better than the GTX 550 Ti, which is pretty much on par with the GTX 650.

akshayghai29 is looking at the retail GT 640, that is DDR3. The GT 640 OEM GDDR5 is interesting; it is pretty much identical in every way to the GTX 650 except with somewhat lower clockspeeds.

So yeah, if you have a GT 640 DDR5 OC'ed to 650 clock speeds, then you'll be roughly on par with the 550 Ti. However,
this doesn't really matter; 550 Ti, 555, 640 and 650 all SUCK and you should aim for at least a GTX 660 or HD 7850.