Hello, i want to install the latest cuda toolkit (that is 7.0) on my laptop (Ubuntu 14.04), which drivers should i have in order to work properly?
The CUDA toolkit runfile installer should have the necessary driver for your GT640M.
If you want a different (e.g. newer) driver you can select one from www.nvidia.com and download the runfile installer for it, and load it after loading the CUDA toolkit.
If you use the package manager method, the landscape is a little different. Be sure to follow the CUDA 7 linux getting started guide, and note that ubuntu notebook installs may be a little bit more involved. There are many questions about it in this forum that you can refer to.