I've heard that the allergens that cause people to be allergic to cats are also found in the saliva of cats and that is why their fur is bad. Because they lick their own coats and it then carries the proteins around the house.
So, I think that if you have a cat like the Sphinx, it would be pretty hypoallergenic, because it doesn't shed so much hair (the Sphinx is that naked looking cat, although they do have a very short, velvety coat).
I often see rag doll cats on lists of hypoallergenic cats but I'm not sure if they are or not. They are certainly a lovely breed anyway.
I would suggest if you are going to buy a cat and you are worried about allergies, then ask the breeder to let you hold him for a while so you can see what happens. That's the only way to make sure your new cat isn't going to give you the sniffles.