What's all the numbers for?
The numbers d300 d700 are camera bodys, -200, 70-200 are the focal length of the lenses. 1.4,1.8, 2.8 are different aperture lenses, they enable you to take pictures were you would struggle with a kit lens of say f4.5 to f5.6, you will get good pics with the kit lens in good conditions but on cloudy darker days, low light, high speed action pics you would struggle thats when such as the 2.8 zoom lens will benefit you, it can stay at 2.8 through the full focal length while the cheaper lenses struggle for light the 2.8 will probably do the job. Hope this helps, i am