I will reply all your recent questions in this thread.

- The difference between 50mm 1.4D and 50mm 1.4G isn't very much. The latter has SWM (silent wave motor) so it is more silent than the previous one. Also, in optics and sharpness 1.4G beats its predecessor (although not by much). I'd recommend 1.4G with little doubts (because the autofocus speed in G is a little slower than D). Still 50mm 1.4G is what you should buy. Now if you want to save some money and buy that newly released 58mm 1.4G that nikon just released, be my guest, but it will cost more than your camera.

- Filter lenses are used for many reasons, and it depends on what kind of filters you are trying to buy. But if you are buying a 50mm 1.4G, you need to, without any doubt buy a 58mm UV filter the moment you purchase the lens. 58mm is the diameter of the lens, hence 58mm filter. This will not only protect your lens from UV rays but also from scratching and other damages. Remember, lens is but just a glass, if you drop it or scratch it, all your 500$ will flushed down the drain. Now there are many choices for 58mm UV filter as well, there is a high end filter from Singh Ray and a little less (but expensive) B+W, but I'd ask you to refrain buying from them. Get a Hoya filter, doesn't cost much and serves as good.

Also, there are other types of filters, warming filters, circular polarizing & linear polarizing filters and neutral density filters, and many others. For your 50mm 1.4G, you do not have to worry about any of these. Get back to me when you buy a wide angle lens in future and we can discuss the possibilities.