Cats and Toxic Houseplants

Sorry if this has been covered before. I tried to do a search, but it's down again.

I was just wondering... I've had cats all my life, but have only recently gotten into having plants. I keep reading about all the toxic and poisonous plants out there, and it's scaring me! I have 2 english ivies, a Marble Queen pothos, and 2 heart-leaved philodendron vines that I thought were Green Jade pothos plants when I bought them, (I also have a zebra plant and a brand new gardenia, both of which are supposed to be non-toxic). The first three are high up and out of reach of the cats, but they have access to the philodendrons and the zebra plant.

Has anyone had any experience with their cats eating a plant listed as poisonous, and then getting sick? How much do they have to eat before they get sick? I mean, it seems like just a nibble, for the most part, won't do anything. It sounds like it's when they chow down that they start to get symptoms.

My cats completely leave the plants alone, even the 2 that are easily within their reach, so I don't think I'll have to move them. The one cat I have (Abby!) that seems to like to chew things only chews things that are hard, like the antenna to my cell phone. I actually put her on the table right in front of the philodendron when I first got it, and she sniffed it but otherwise couldn't care less. I've been inspecting the leaves for the last couple of weeks (ever since I got them), and haven't seen any teeth marks or damage. How worried should I be that a little nibble might hurt my cats?

Kelly