If you have a slow cpu, buying top of the line gpu, wont do you much good, because your slow cpu, will bottleneck the performance gain from the gpu.
So in a sense, yes your cpu/processor, does effect what type of card you "should" get.
Same with motherboard, the option to SLI/Crossfire (buy more than 1 graphic card, and via cables use them together) is a thing that should go into consideration with buys. Usually motherboards cannot do both SLI and Crossfire, so you get one or the other (which then plays a small part in consideration of what card to get).