I am using wordpress twenty ten theme in my another blog. I was facing a difficulty to increase the width of sidebar of the twenty ten theme. Actually I want to insert one of the adsense ad there and it was not possible to do the same with the existing margins of the theme. I searched a lot and experiment a lot. At the end I was successful. I want to share the same that how I increase the width of sidebar of twenty ten wordpress theme. Here is the method:

Step 1: First login to your blog and go to the dashboard

Step 2: Go to the appearances

Step 3: Look for Editor in appearances. You may be in the style.css page

Step 4: Look for the lines

#container {
float: left;
margin: 0 -320px 0 0; (Change your existing setting to this one)
width: 100%;
}
#content {
margin: 0 340px 0 20px; (Change your existing setting to this one)
}
#primary,
#secondary {
float: right;
overflow: hidden;
width: 300px; (Change your existing setting to this one)
}

Step 5.Update it.

I hope now that the there will be increase in the width of sidebar of your twenty ten wordpress theme

You can check the same in my second blog http://managementation.com/brand-strategy-decision/

All the best.