32bit.
It uses OpenGL. I think the minimum shader level required is 1.5, but I could be wrong on that. Shaders definitely requires at least 2.0, and real-time raytracing definitely requires at least 3.0.
Not that that should be an issue, if you have an AMD/ATI/NVidia card from the past 5 years, they should be able to handle the OpenGL requirements without issue. They may choke on some of the other stuff, but they should have all the features.