As your school slides down the top 100 list, your grades better be more impressive.
Once you get to the interview stage, however, the Harvard vs. ____ undergrad degree becomes less important, especially the further you are in time from when you graduated. Getting an interview is half the battle and is a critical advantage, but if you went to Harvard or Stanford and you're a bad interview, you won't get the job.
If candidates have advanced degrees, undergrad degree makes almost no difference at all (unless you went to BYU and the guy screening your resume is me).
I've literally looked at thousands of resumes and interviewed a LOT of people.