In addition to Stefans answer: If you use centered columns with a fixed width quite often, you can define a new column type:
The "x" column takes its width as argument. In your document, you would write e.g.
EDIT: Thanks to an answer from Ulrike Fischer on the German-speaking forum mrunix.de, I learned that there is still room for improvement: One might want to manually break lines within a cell of a "centered" p-column, but
\newline doesn't work correctly in this case. Solution: assign the meaning of
\\ (as defined by
\newline within the cell: