I work at a software company. I interview often.
Wear a suit.
Minimum: Nice shoes, Khakis, dress shirt, navy blazer, and fucking tie (this is the most basic casual dress combo you should be able to wear anywhere-- interviews, weddings, funerals, happy hour, clients, etc.)
I don't care how low level IT it is, if you aren't responsible enough to handle a jacket and tie, you aren't responsible enough to handle my servers. Maybe it's just 'details', but "I.T." is about "details".
I worked for a company that wouldn't even interview you if you didn't show up in a suit.