Thanks for your answers.
later i found out that the compute capabiliy 3.5 wich is really rear among the nvidia cards, doesn't really say anything about the speed, just about the supported technology.
So the particular 640 of interest allows to test the cuda 3.5 stuff, but unfortionatly very slow.
So i guess ti would be like a wheelchair having the right specs to fit a F1 tire. yes you can test if it works, but not really anything interesting about speed up. Because i wanted dynamic paralellism (only cuda 3.5 and up) i guess i have to save some more, and be very lucky on ebay, geting a used titan.