Hey Vance, my wife and I have had to make that decision with 2 of the 4 cats we've "lost" over the past 28 years of our marriage.
(All kitties mentioned are in our "Our Cats" photo album at:
http://www.medhelp.org/photo_collections/list/1847?page=3&personal_page_id=40606&photo_collection_id=1847 )
We adopted Queen Victoria Grey (aka Bear 1st) and Calamity Jane Grey (aka Tookie) as 2 month old kittens in February 1983. Bear died in her sleep of a heart attack after a short bout with a lung infection at age 17 1/2. Tookie, we made the Decision when she was 21 1/3; she'd had 2 strokes and the 2nd one just devastated her.
We rescued April Dancer (aka Gremlin) from a parking lot at Eglin AFB, FL, where my wife and I were stationed when she was about 2 months old. She died of anaphylactic shock during a routine procedure at the vet's at 6 1/2. Cried harder for that little critter than anyone, human or animal, I'd lost before then.
Cerridwen Blythe (aka Foofie), we took her in as a stray at a little over 4 years of age; very, very sweet cat. We had to make the Decision when she went into massive liver and kidney failure at about 16 1/2.
Tuna (aka Tuna ;-), I took her in at about 4 months; a stray who came in from the dark...during the evening when my parents were having a picnic to announce my engagement to Anita back in 1978. She was my little buddy and I had to leave her with my parents when I got married (the quarters into which we were moving allowed absolutely no pets). My dad had to make the Decision when she was a little over 16. She had gotten very sick; mom said she could tell he cried on his way home from the vet's. She had become his little buddy.
It's never an easy decision, even when there is good cause and you know it's the right thing to do. Sometimes it hurts more when it is the only right thing you can do. Hold your wife a little tighter, the hurt will fade in time, but the love will last and you'll smile at kitty's memory, which is as it should be for any of our beloved.
take care - Pip