Valerian will have mild sedative effects and has been used short-term to
calm animals.  When used for extended periods of time, side effects include
gastrointestinal upset.  While I do not know what exact dose is considered
toxic, there are expert at the ASPCA's Animal Poison Control Center.  Their
phone number is 888-426-4435.  This is a non-profit center and does not get
any funds from donations to the ASPCA unless they are specifically intended
for the Animal Poison Control Center.  Calls about plants usually require a
$50 payment.
You can also call a animal clinic that practices holistic medicine.  You
will usually find a specialist there who may be able to give you the
information you are looking for.
Here are some links to their site:
Common outdoor plants which pose potential threats and their effects:
http://www.aspca.org/site/News2?page=NewsArticle&id=20269&security=2620&news_iv_
ctrl=1101
List of toxic plants
http://www.aspca.org/site/PageServer?pagename=pets_toxicplants

http://www.fanciers.com/cat-faqs/misc.shtml
Catnip and Valerian.
Catnip is a plant that causes various reactions in cats. Very young cats and
kittens will not be affected by catnip. About 20% of cats are never affected
by catnip. It is not known why or how catnip has the effect it does on the
rest of the cat population. It is a non-addictive "recreational drug" for
cats with no known harm to the cat. There was an article in Science [exact
reference?] on the neurological effects of catnip on cats. It seems to
stimulate the same pleasure centers in the feline brain that orgasm does.
Most cats "mellow out" and become sleepy and happy, others start acting very
kittenish. A small percentage will become possessive of their catnip and may
snap or hiss at you.
You can find wild catnip plants in most weedy areas, and harvest the seed.
Or you can buy seed from companies like Burpees or Parks or Northrup King --  
most garden centers have catnip seed this time of year -- check the "herb"
section. Or even seed racks in the grocery and discount stores.

Catnip is easy to grow. You will need to keep the plant itself out of the
reach of the cats as catnip-lovers will quickly destroy it. The best
strategy is to get some growing, and then pinch and prune it regularly and
give the harvested leaves to your cat. Keep it in its own pot, as it will
spread rapidly. Cats will tend to dig up transplanted catnip and eat it
roots and all, but are much gentler on plants started from seed. The leaves
have to be bruised to release the odor, and transplanting seems to be enough
bruising...

Nepeta cataria is the common catnip; other Nepeta species have varying
amounts of "active ingredient". A good one is Nepeta mussini, a
miniature-leaved catnip that makes a good rockgarden plant. Nepeta is a
genus of the Lamiaceae (=Labiatae), the mint family. There are about 250
species of catnip, plus a bunch of hybrids between species. Only about 10
are available in this country, though.

You can order seeds from Burpee (215-674-9633)
Nepeta cataria B61424 $1.25; N. mussinii B38828 $1.45

Valerian root is an herb with effects very similar to catnip and generally
makes cats a bit nuts. It is however not as readily available as catnip and
perhaps a bit more potent than catnip.

Catnip and Valerian both act as sedatives on humans.

Spider which one are you actually talking about

this is the 'Valerian', Centranthus ruber   Centranthus ruber 'Albus' also
known as red valerian or white jupiter's beard
Drunken Sailor, Fox's Brush, and Pretty Betsy
http://www.highcountrygardens.com/31650.html  this one I can find no info on
poisons about just being a evasive weed I would treat it as toxic

this is Valerian officinalis of the one with white flowers
http://www.indianspringherbs.com/valerian.htm
http://www.shamanshop.net/store/proddetail.cfm/ItemID/45638.0/CategoryID/9000.0/
SubCatID/0.0/file.htm
this is the safe one that you can buy over the counter and I posted some
information about in below

I am surprised that your cats don't get a little wacky on it

Matthew