Nikon d40 and sb600.shutter only going to 1/500?

When i have the sb600 flash on the camera it will only let me shoot at 1/500… In most cases thats fine but i want to shoot low key portrait and need a higher shutter speed… How can i change it to let me use higher shutter speed?

You are lucky you have a D40. All the other cameras flash sync at 1/200th or 1/250th seconds

Low key portraits are usually done using a dark background and a single incandescent lamp with some kind of appropriate light modifier to place the light just where you want it with the "house lights" dimmed or turned off all together.

Sample

http://www.modelmayhem.com/portfolio/pic/5060978#5060978

You'll have to adjust your exposure another way. Like fhotoace said, most will only sync up to 1/200 or 1/250.

Use your other settings - bump down the ISO and crank up the aperture (if you can) and change your light source itself as well.

I recommend you to not use flash for low key. The cheapest way is to use a l.e.d torch(flashlight).
check out the link i just used l.e.d torch for this.