Photography - NIKON D5000 camera ISO?

I want to do night street photography with the D5000.
What is the highest ISO I can use BEFORE the pictures starts to get grainy?

The best you can do is to see what your camera is capable of in the environment. Just remember that ISO is for ambient light. It all depends on your depth of field, shutter speed and F-stop. Do a few test shots and see at which setting the photo becomes too grainy for you. Also depends on the look you want to create.

Actually, the correct term is "noisy" (as in digital noise) since "grainy" refers to film.

Per this review - http://www.imaging-resource.com/PRODS/D5000/D5000A.HTM - the D5000 does rather well at ISO 1600. You'll have to do a few tests yourself to determine if the digital noise at ISO 3200 is acceptable or not. (The reviewer said he found ISO 3200 acceptable for 8x10 prints.)

Another consideration will be the lens you're using. If all you have is the 18-55mm f3.5-5.6G zoom then your ISO is going to have to be high if you want to avoid blurry images caused by subject movement. If your budget allows, invest in the Nikon AF-S 50mm f1.8G prime lens. Since f1.8 is a full 2 stops faster than f3.5 and 3-1/3 stops faster than f5.6 you can use a lower ISO and still get a shutter speed fast enough to avoid blur caused by subject movement. Use of a tripod is strongly encouraged.

I use and recommend this site for low-light exposure calculations:
http://www.calculator.org/...osure.aspx

I used the Scene 'Distant view of city skyline or floodlit buildings' and ISO 200 for these pictures:

100mm @ f11, exposure for 30 seconds.

200mm @ f11, exposure for 30 seconds.

Although those pictures certainly aren't what you are trying to accomplish, I believe they show how accurate the referenced site is.