you should think about what you like most --- dealing with networking issues or dealing with computer issues ...

if you prefer the networking side, do as rhfromm suggested ...

if you prefer the computer side, look into getting certified in solaris, hp-ux or aix ... linux is also good but the majority of corporate it environments require that you know one of the three main unix flavors i mentioned ...

or you can combine both ... get certified in both a unix flavor and in networking ... this way you have more to bring to the table when you talk to recruiters ...

good luck!