I swear by the New Healthy Living Air Purifiers/Ionizers. I used one when I lived in CT and had 6 cats and it was great. It's a bit expensive, but you do get what you pay for. They are silent and adjustable to the room size.

Unlike some of the purifiers, these come with glass filters that you simply wash, air dry and put back in the unit. I'd try ebay. It'd be a little cheaper than going through a dealer. The dealer I went through was charging $500.