I believe that the graphics card is generally abstracted by OpenCL or OpenGL support on Linux. So applications that support OpenCL/GL can make use of your graphics card for processing.
I know for sure that darktable and hugin can both use OpenCL. I haven’t done this much with darktable, but with large hugin projects, my GTX970 significantly reduced the processing time.
Lots of Linux users don’t like nVidia because they don’t support the in-kernel drivers. If you want decent performance with an nVidia card, you’ll need to use the proprietary driver. Good news is that on many distros now, Ubuntu included, but also Fedora and OpenSuSe, it is very easy to install the proprietary driver.
AMD is working towards getting a good in-kernel driver implementation, which when finished, will be quite nice. Right now those drivers require a very recent version of the kernel.
Graphics cards can be quite expensive, so as long as it supports OpenCL/GL, then the “best” one to get is one that won’t break the bank 
I’d rather have more RAM than step up to a better graphics card, but that is my opinion.