Thank you for your replies.
I have experience with Matrox and ATI cards under Linux and they work good enough for me. I had a nVidia card a few years ago but it was not sharp enough for 1600x1200 resolution. The explanation I got from my supplier is that the nVidia card was a good 3D card but not very good for 2D.
I have checked both www.x.org
but couldn't find any compatibility lists. Maybe I didn't try hard enough?
Both ATI and Matrox have drivers for Linux but the big question remain, which company is most Linux friendly? I realise that my action in this case will make very little difference in the graphics card market but if all of us try to buy the products with the best Linux support things will change.