1.I had to choose between GT240 or 9800GT; what is the best for CUDA?
What I need it for ? I need it for recovering rar password and WPA wifi with BeckTrack OS
2.Both of it work with windows 7 right ? just in case I need it for game
3.I hope there is table give numbers of password per second for GeForce cards from the top to the lowest
Yes, I am sorry, it is possible. Though GT240 has more features, so particular application could benefit from it. And I am still suggest to go for GT240, cause it is newer so less troubles in general.
9800GT has twice lower register number, so if application is tuned for GT200, it could work worse on 9800GT than it actually could.