By not getting the correct exposure from camera you're already introducing some noise into the picture anyway
D3s / D3x / 18-55 f3.5 - 5.6 / 55-200 f3.5-5.6 / 50 f1.8
Use a slightly slower shutter speed since the use of flash will help to negate a little bit of motion blur. Try 1/30 to 1/60
Tilt the flash head with bounce card on a little bit towards your subject and up the flash power. Or just shoot with direct flash. Try and see which u prefer.
Lastly practise more.
I'm not going to suggest anything about getting a different camera because that's not going to help you improve your shooting skills.
If you're going to underexpose in camera then increase exposure in PP you'll introduce noise to the picture
with this type of event... you have better luck with a faster lens.
You can never get a pleasing effect with a flash in this type of environment, regardless of diffuser.