is bandwith is very important for gpu i have 28.5 gbps bandwith is it okay

It really depends on what you have for a gpu. 28.5GB/Second as you put it is not enough for any modern discreet GPU. Some APU's or Intel CPU's with a built in gpu have around that much bandwidth available for the gpu but for the AMD APU's they get much better FPS if you overclock the memory increasing the bandwidth. Highend GPU's nowdays can have over 250GB/Second bandwidth available to them with some going over 300GB/Second.