Where do commas go in relation to quotation marks?

Inside. Always inside:

He said he values most a computer's "speed," and I totally agree!

In fact, not just commas but also periods go inside quotation marks. Here's an example with the period inside:

The most important characteristic of the new computer, he said, is "speed."

You can make a pretty good case why the period and comma shouldn't go inside, but just look at any book or magazine published in the United States. You'll always see the periods and commas inside the quotation marks (unless the editor was asleep!).

The convention is strong: inside!