I really wish there were more widely available information about HOW toxic the various plants are. For example, Impatiens is on that toxic list, but I've also seen it on "non-toxic" lists and I can assure that Stormy has eaten several impatiens plants gradually, leaf-by-leaf, with no ill effects.
Similarly, after reading that English Ivy, while toxic, is pretty much the may-upset-kitty's-stomach kind of toxic, I let my ivy live in the apartment for a month while my office was being finished. Stormy ate her share with no ill effects (I mean, she barfed a lot, but she didn't barf more than usual. Stormy always barfs a lot.). At least, there were no ill effects to Stormy -- the ivy is now in my office and is a little the worse for wear.
It also depends on how prone the cat is to eating plants. We used to get Easter Lilies all the time growing up, and while I would never recommend bringing a true lily into a house with cats (they're just too poisonous to mess around with, but we didn't know that then) we never had any problems along those lines: the cats we had when I was a kid just weren't plant-eaters. While I wouldn't bring something like a lily into a house with a cat (note: peace lilies are not actually lilies and are much less toxic) the level of toxicity I'd be comfortable with would depend on the cats plant-munching habits.
I also gather that things that are toxicity problems for dogs are not always problems for cats. Dogs will do things like eat entire plants, entire bowls of chocolate, dig up bulbs and eat them, that sort of thing. Cats, not so much.
I've had an amaryllis for years and even with Stormy it hasn't been a problem: the toxic part is the bulb and she's not likely to dig that up and eat it. (Actually, she doesn't really eat the leaves much, either. I guess amaryllis isn't as yummy as impatiens is.)