In order for you to play Torchlight II on Linux with minimal graphics settings, your computer must meet the following requirements
Operating System:
Ubuntu 12.04 LTS (or similar, Debian-based distro)
Processor (CPU):
x86/x86_64-compatible, 2.0GHz or better processor
Memory (RAM):
2GB System RAM
Hard Disk Space:
1.7GB free space (subject to change)
Video Card:
OpenGL 2.0 compatible 3D graphics card* with at least 256 MB of addressable memory (ATI Radeon x1600 or NVIDIA equivalent)
Internet:
A broadband internet connection (for Steam download and online multiplayer) - Online multiplayer requires a free Runic Account
*See Valve's steam-for-linux github readme for further information about Linux video driver support