@Hans
Knowing the Tabor bus bandwidth what is the best gfx board suited to it. By best I mean fastest even if for now the DMA transfer is not utilized.
Kamelito
The higher the clock speed the faster you software will run. This may change over time as Warp3D Nova grows and newer software starts to take advantage of the greater number of texture and shader units available on these cards. Older software will still be dependent on higher core clocks.
At the moment the R9 270X cards are the fastest since they have higher core clocks than the otherwise more powerful R9 280/X cards.
For the Tabor I'd recommend getting the fastest compatible R7 250 card you can get since some of them have higher clock speeds than the R9 280 cards. They also use a lot less power, are smaller and generate less heat and noise making them better suited to smaller ITX style cases that most Tabor boards will find themselves housed in.