Taking photos of an indoor party?

I have my own studio photography business but have just been asked to photograph a party in a hall. I have a Nikon D90. What would be the best lens to use for this? And do I need a speedlight flash?

17-55 f/2.8 should do it.

Take a fast glass if you have it. F2.8 or lower. You should bring a combination of lenses that allows you to cover different focal lengths from wide angle to telephoto approximately 17mm-100mm. A fisheye lens would make for some nice shots as well. Use a speedlite that can rotate and pivot to bounce the flash off the ceiling or wall. Point the flash head over your shoulder when shooting. Do not bounce the flash if the wall or ceiling is colored. In that case you will need a flash diffuser of some kind in order to get the best quality of light. If the hall is well lit you may not need a flash if your lens is fast enough and you can adjust your iso without introducing noise. The darker the hall the more likely you will need flash.

Bring a main camera and a secondary/backup.
Bring 2 TTL flashes.
Bring a 24-70 f2.8
Bring a 50mm f1.4

That should be all you need to cover a party in a reception hall. Hopefully you know how to bounce your flash.

If the ceiling inn't appropriate for bouncing you may want to bring a bracket and a small diffuser. If you want to mix your flash with the ambient, having a set of gells on hand would be useful to properly match the light sources.