Which would you prefer? The AF-S DX NIKKOR 35mm f/1.8G or the Nikon 50mm f/1.8D AF Nikkor?

Which of these lens are better in portrait and street photography? Which one has better pictures in low light conditions?

Couple of things to consider: The 50mm 1.8 will not autofocus on the D40, D40x, D60, D3000, D3100, or D5000. Otherwise it works just fine.

They are equally good in low light. The 35mm is probably better for most people as a general-purpose lens, but is too wide for a head and shoulders portrait. Get the 50mm for that.

I have both and use them roughly equally on my D90. I might use the 35mm a bit more, though

For portrait? The 50 mm lens.

For street photography? Something much wider that a 35 mm "normal" lens. I would just use the 18-55 mm you have already. If your budget can stand it and you really want a prime lens, the 20 mm f/2.8 would be my choice.

I would not shoot portraits with a 35mm tends to exaggerate facial features. Apart from that they are both optically very good.

What I tell all prime buyers, if you got a zoom that covers the focal length that then just use that to try it out, see if you like the focal length.