Originally posted by Sarahbell 
Ivan, can I just ask why the x setting is best to use? Is there a distinct advantage of it compared to using the manual setting? I, honestly, never even noticed that x on the dial before!
No real advantage. X locks shutter speed to 1/180, that's all if I can remember well. You can use M and set shutter speed to 1/180 or below.
Now, reading you original post again I would actually recommend M mode combined with slow shutter speed for conditions you need. Remember, by varying aperture you can control flash exposure (at fixed flash power), while shutter speed controls ambient light exposure.
You can start with 1/180 and find proper aperture for selected flash power and flash-to-subject distance (assuming manual flash power is set). Then go down to slower shutter speed to capture ambient light, dimly lit clubs may require 1/30 or less. Flash will freeze the motion so 1/30 should not represent a problem. If you go to very slow shutter speeds, say 1/2, you may overpower the flash which would result in a combination of sharp image from the flash discharge (flash burst is around 1/1000s) and another superimposed and often a bit blurred image from 1/2s ambient light exposure. Makes cool effects sometimes, give it a try!
Just never use any camera mode other than M or X for predictable results with portable or studio strobes. Combining P-TTL flash mode with M or X camera mode is also fine (read distance scale on the flash for given aperture!) but I prefer full manual control over both flash and camera.
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