Get the 50mm 1.8 or 85 1.8, but to pick one i'd say the 50mm.. i've shot with the 1.4s before and honestly the difference between the 1.4s and 1.8 isn't worth the price hike. Especially with the 85mm. I believe you can get an 85mm 1.8 on
B&H for something in the 600 range and a 1.4 is twice that, not worth it
The Nikon 85mm 1.8G lens goes for $497 as listed on Nikon's website. I picked this lens up a couple years ago when Nikon started offering their "instant rebates" on select lenses and picked mine up for $397.
I have both the 50 and 85mm 1.8G lenses and use them both regularly on my D3100, and D7000. I like them both for different reason's, the 50 is good if you want to add some atmosphere to the portrait by using the background. The 85 really works great for isolating the background both from the reduced field of view as well as the longer focal length and fast aperture.
Edit: Apparently the retail price for the 85mm 1.8G has dropped a bit as Nikon's website now list it for $449.95. The 85mm 1.8G still comes in at more than three times that at $1699.95 which is absolutely ridiculous.