I'm using Debian 7.7 with the proprietary NVidia driver 319.82 and a GeForce 560Ti. This drives 2 portrait (rotated) monitors over DVI with no problem and I'm happy with the result.
I'd now like to add a 3rd portrait monitor. It seems like one can't mix NVidia and HD3000 GPUs, so presumably I'll have to buy another NVidia card. So which NVidia will work? Can I just buy any random cheap NVidia GPU (e.g., GeForce 210) and have it work? Or are there only particular combinations that are Linux-friendly?
(I'm not terribly concerned about rendering performance, OpenGL compliance level or SLI ability, so hopefully any card will do, and the only concern is compatibility. My only real requirement is that it can do 1200x1920 rotated over DVI, which I guess any modern-ish one will manage.)