[yohui]

The other replies cover what's best for machine learning, but as for your terminology question:

- Pascal: codename for Nvidia's latest microarchitecture, used in the new GeForce 10-series announced today. (AMD counterpart is "Polaris".)

- GeForce: Nvidia's consumer graphics card brand. (AMD equivalent is "Radeon".)

- GTX: label for GeForce graphics cards meant for gaming and other demanding tasks (see AMD Radeon "R9").

- Titan: special name for super high-end GeForce graphics cards. (For AMD, see "Fury".)

You might also encounter Nvidia Quadro graphics cards (comparable to AMD FirePro), which are meant for professional workstations, and Nvidia Tesla graphics cards, which target high performance computing.