I would recommend distilled water (it's plastic safe) . 100% cotton cloth or micro-fiber.  You can also vacuum it off first (soft brush attachment). If cleaning the dust from the inside  ... take outside and blow-out with air-compressor at 40-50psi. Be sure to get all fans, the video card, the power-supply, and the liquid-cooler's radiator.
Dear community, I have a tip for cleaning your Alienware case outside panels. Take a industrial Alcohol Propanol from at least 70%, make sure you have a can with a sprayer and mix 100ml Propanol with 150ml water, spray 8x = 0,25ml on a clean and soft cloth and gently wipe all panels. I have a white R 13 and it looks amazing after 3 months, so bright!  
Cleaning tip for Alienware Aurora's