<p>What you need to find out is how long is the fellowship for, and what are the terms? Are students expected to have a research or teaching fellowship? Some grad schools, such as Princeton, give a full fellowship for 5 years. Many others give a full fellowship for the first couple of years then expect students to be research assistants or TFs; others expect them to be TFs from the first (and this also can vary by department).
Also, keep in mind that stipends are taxable.</p>