Here's what I recommend:
1) For white frosting, use a simple confectioner's sugar and butter (with a little vanilla). You can find a recipe just about anywhere. Alternately, cream cheese frosting is always great - not just on carrot cake.
2) For chocolate, I advocate this recipe over ganache. It is simply more satisfying:
1/2 lb butter (two sticks)
4 oz Unsweetened chocolate
2 ounces Bittersweet chocolate
1 teaspoon vanilla
1 pound confectioner's sugar
8 ounces sour cream
1/4 teaspoon salt
Method:
Melt butter and chocolate together (note: take no shortcuts with the chocolate! Buy the best you can find). Allow to cool for 15 minutes
Add in confectioner's sugar and mix until smooth with NO LUMPS
Add in sour cream, vanilla, and salt (if desired). Do not overmix as it will lighten the color of the icing. Allow to cool completely before using.
I almost never use a pastry bag when icing cupcakes. I prefer to use a teeny tiny offset spatula for a smoother look. If you want to ice with a pastry bag, I'd recommend a large star tip, say, # 2 or an 11mm round tip (slightly less conventional). I like to use pastry bags and tips for garnishing cakes once they are iced...which you most certainly can do with cupcakes, too.
One thought: I have cupcake parties yearly, and what I do is bake assorted flavors of cakes and make two icings: chocolate and white. I then mix color into small batches of the white for 'drawing' - I make small pastry bags from parchment and cut a round tip. I put all kinds of garnishes on the table, from gold leaf and gold dragees to various colors of sugar and edible pearls. I allow guests to ice their own cupcakes - and most have a great time with it.
CONNOISSEUR, n. A specialist who knows everything about something and nothing about anything else.
-Ambrose Bierce, The Devil's Dictionary
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