I'm making a chocolate cake with chocolate icing covered in ganache. I've never used ganache before. Does it harden or does it stay tacky. And does anybody have a good recipe for ganche?

I got my recipe from the Cake Mix Doctor book. It is very simple! It does harden pretty fast. I drizzled ganache down the sides of a cake I did (pics in my photos) and I had to keep adding more to get it to drip further down the side.
I LOVE using ganache. I have done quite a few cakes since then with ganache on them!

The easiest way to make ganache is melt in a double boiler equal parts of chocolate chips and whip with a tad of butter for shine.

You can pour as is (when cooled a bit) on the cake, or you can let harden in fridge for an hour or so, and then it becomes spreadable as an icing.

Enjoy! Ganache tastes way better than buttercream!

When I pour it down the sides it won't affect the BC icing at all?

You can let the ganache cool to about room temp. This will help with a couple of things - it won't melt the icing undernieth, AND it will be a little bit thicker for better coverage without having to recoat.

If the ganache hardens or stays tacky depends on the recipe, specifically the ratio of pure chocolate to the fat (whipped cream or butter).

This recipe is from my most favorite chocolate book, Ultimate Chocolate by Patricia Lousada:

Chocolate Glaze
3 oz. bittersweet chocolate
3 oz. semisweet chocolate
1/2 cup unsalted butter
1 Tbsp. corn syrup

Chocolate into small pieces - melt choc with other ingredients in heavy saucepan (keep heat fairly low to avoid burning). Stir occasionally until melted.

That recipe sets up nicely, not too soft and not so hard that it snaps and sends pieces of choc. flying when you cut the cake.

Let it cool until it's no longer warm to the touch. If it gets too thick to pour, heat it up slowly. I like to double or tripple the recipe above, just to have plenty available. You can freeze and fridge leftovers. Just make sure no condensation gets in when reheating.

If you set the cake on a cooling rack that's over a cookie sheet with edges, you can catch the excess choc to reuse too.

I may be too late to help you out on this, but I highly recommend the chocolate ganache 1 recipe on this site....also when you pour it over the buttercream you will want to make sure that its not hot....you have to let the ganache cool somewhat....what I normally do (and you can see my two ganache attempts in my photos) is pour it when its pretty well cool thick but still will run down the sides...and then when i get it just about where I want it, I pop it in the fridge for 10 minutes or so to help set the ganache....It doesnt harden......it is just sooooo good!