I would like to upgrade my machine I have a 960 gtx 6go geforce that arrives quickly within its limits I wanted to know if I add a second houdini graphics card will use it also in the viewport or not at all?

In the viewport, not at all. You can use the second card for openCL compute though.

All right so the best thing would be to have a big map with a lot of ram in main for the viewport?