Gallium (which is now part of the Mesa project
) is a layer that sits between the 3D graphics API and the DRI hardware drivers, much as Mesa has always worked.
There are good free/libre 3D drivers for a number of legacy cards and for ATI/AMD Radeon cards.
The situation with Nvidia cards is that there is a project underway to implement 3D drivers
, but there is still some way to go. The current nouveau driver is quite usable, but doesn't yet have the wide 3D support that you will probably want (it is reasonable for 2D acceleration). The proprietary Nvidia drivers do not fit into the Mesa framework, and Nvidia have repeatedly said that they do not intend to open their drivers (let alone firmware). The open source alternatives will take some time, as they involve some complex reverse engineering.