If this is traditional night club then the following information applies, if it's a musical band, like a concert in a club it does NOT apply.
I would not recommend you spending ANY money on a lens to shoot in a night club .... for now....
I'm don't know what lens you have, but are likely to have at least something around 50mm.
The key in nightclub photography is NOT just the lens, it's mostly the flash technique, and you do have a good one !
I started in nightclubs with D70 and the same flash + the kit lens and that was plenty enough.
There is no need to have a fast lens, because you will want to shoot at a slow speed - sometimes down to a half second handheld. The action will be frozen by the flash.
Try it at home first in the evening, with just a few lights on.
Later on, you may want to go with a wide angle if you do not have one already. It's a big plus to give power to your pictures, and because clubs are crowded so you can't step back too much.
Of course, a f/1.2 or f/1.4 lens will help doing some DJ shoots and so on - but 99% of my pictures are NOT shot so open. My guess is that most of them are shot around f/4 or even more.