Cat Problem
I second what Sudsmaster says about the ionic cleaners in CR reviews. Too many negative ions in the air are not good for you, and they dont seem to remove enough "stuff" to be effective.
I believe they rated the Freidrick (maker of air conditions) air-purifier to do fairly well but they are about $350.
I am not allergic to cats but having an indoor male short haired american cat, I can tell you about others reactions to my little fatso. I love Electrolux too but they dont clean as well as some other machines and even the Gaurdian (to the best of my knowledge) is not a sealed system unit.
I had two Mieles, a Red velvet and blue moon, and they are a sealed system, but my friends still had an allergic reaction when they came into my house. I also have a Kirby Diamond Edition and only use the newest HEPA bags in it but it doesnt make an impact either.
I will tell you that I am OCD about how clean our house is kept. Since I put in our Vacuflo central vac (bagless and vented to outside), our friends seem to not be effected as much by the cat. It takes much longer for them to have any kind of reaction to the cat dander, and one of my best friends sometimes doesnt have any reaction. Of course nothing will change the fact that you have a cat, but there are ways to help the situation. I know a central vac my not be practical as it is a large investment, but its worth a thought.
My other suggestion is to check out a new Rainbow. I have never used one but if i didnt put in a central vac, a Rainbow would have been my next machine. Maybe its in my imagination, but the fact that you dont have a bag -with cat fur in it for air to pass through everytime-, no dust storm when emptying it like a normal bagless, and you start with fresh water each time, will make a difference for allergy problems.
Anyway, just my two cents, but good luck! I hope you find something to help your allergies, I could never part with my cat and I hope you dont have to.
-Steven