I used to be a smoker in the room, so I tried out a bunch of purifiers to cut down on the smoke and smell...
I have the Holmes model that was mentioned above for the bathroom, and I honestly think that it didn't make a bit of a difference, and it was just too noisy for me. I honestly think you would have a hard time sleeping in the same room with it...
My office had a bigger unit, http://www.amazon.com/Holmes%C2%AE-H.../dp/B000C173W0
, and it worked great for awhile, but since I stopped smoking, I no longer needed it.
First things first, all HEPA type purifiers need regular filter replacement, which will get costly. Not only was it difficult to find the filters after about a year or so in service, the larger units require bigger filters which cost around $75 a pop, and depending on usage, it might require replacement every 6mo to a year.
If noise is a issue for you, and you do not want something that needs filters, and just require something to help you sleep better at night due to dust and allergens, then look into investing on one of these...
Its a bit costly at first, but you will end up making your money back and then some after a year or so since you do not need to buy filters for them.
Also, its the quietest way to go, and especially if you place it near your bed, it will certainly be great at what you are looking for.
Hope this helps!