In that case, TS, I recommend u to stick back to your 18-55mm. Not that the kit lens is lousy, but think: do you need to isolate your subjects from the background at 18mm (27mm equivalent)? Are you going to take landscapes and blur the background, leaving only the foreground visible? At first I thought maybe u wanted to take street/portrait photography and needed that sharpness all over and a 50mm 1.8G or 35mm 1.8G DX would be justified. But landscapes...
At first I had the same misconception as you. f1.8 prime lens = sharper than f3.5 zoom lens. So i bought the 50mm f1.8D. And I realised my 50mm f1.8D became a burden coz i had troubles with composition (either too far away or too near from subject, cant zoom to compensate) and i didn't need to use f1.8 aperture in bright daylight. So now when i shoot landscapes i revert back to my 18-105mm kit lens, stopping down the aperture to increase sharpness across the frame for landscape shots. My 50mm f1.8D I only use it sparingly, when i already pushed my D90's ISO to 2000 and still too dark, then i use the f1.8 aperture for night shooting.
Final words, dun buy any lens yet, save up to buy a good lens. If u r into landscapes, maybe u can save up to buy the Sigma 10-20mm or the Sigma 8-16mm or Tokina 11-16mm, that would be worth the money invested. Meanwhile, work around the apparent restrictions 'imposed' on you by the kit lens. Maybe when u can overcome these limitations u become a better photographer with heightened awareness of composition and lighting. Remember that cameras in the film era do not have the luxury of the preview screen and the SD card you now enjoy on your D5100, and that's where they hone their skills instead of relying on the camera to provide the shots for them.
