Nikon d90 which lens to buy for portraits?

I heard a lot about the 50mm f1.8d and 1.8g which are awesome for portraits but can someone explain to me what are the differences between those two and the 50mm f1.8 one (which i heard is half-price but a lot worse)?
what i'm looking for is a camera for portraits with a good focus sharp quality and especially for low light photos

Nikon offers two 50mm f1.8 prime lenses. One is an AF model which will auto focus on your D90 and the other is the AF-S model which will auto focus on the D40, D60, D3000, D3100, D5000 and D5100. The AF model won't auto focus on those models. They are the same lens except the AF-S model costs about $100.00 more.

The 50mm 1.8 D is all you need, because the D90 has its own built in motor drive for the autofocusing.

The G version was made for newer bodies that don't have an integral drive.

The quality of the 50mm 1.8 D is exceptional for portraits. I've had one for about 20 years. Still going strong.

You will be good with these cheap primes. They are sharp.

50MM F1.4D
50MM F1.8D

The 50MM F1.8D is not worse but excellent too.
The other cheap excellent alternative is the 50MM F1.8G

You can also use the cheap macro lenses for portrait and they are sharp,
85MM F3.5G DX Micro (if you have enough room)
40MM F2.8G DX Micro

While the 50mm is not a bad lens, the superior 85mm focal length will provide for a better portrait lens.

A lot of people think that the 50mm lens, after considering the crop factor will result in a 75mm lens. This is not really true. While the crop factor will give it an appearance of a 75mm lens, the lens still retains the characteristics of the 50mm focal length.

For portraits this is sometimes important if you are trying to obtain a shallow depth-of-field. The 85mm lens is going to be superior to the 50mm lens for this, crop camera or not.

Check out the newly announced Nikon 85mm portrait lens ($500).

This might mean you may have to backup a bit to compensate for the crop factor, but the advantage of working with a shallower Depth-of-field capability outweighs this need.

The 50mm f/1.8 G is the better quality lens. It is actually designed with an extra piece of glass in the lens (called an aspherical element) that helps increase the contrast and resolution while reducing unwanted qualities (coma and other aberrations).

Both will autofocus on your camera and both will take good pictures, but the updated G version is the better choice.