$\displaystyle x_n \stackrel{n \to\infty}{\longrightarrow} x_\infty$
One advantage of \overset over \stackrel is that there is a corresponding command \underset, so that you can write $\displaystyle x_n \underset{n \to\infty}{\longrightarrow} x_\infty$. You can even combine the two, to get things like $\displaystyle x_n \underset{\text{Moo}}{\overset{n \to\infty}{\longrightarrow}} x_\infty$ (if ever you should wish to).