I attended a top undergrad and I think it helped, but not in the 'holy smokes she went to X school, let her in!' type of way.
I did ok at my undergrad, and ended up with a 3.4 but got into my instate med school anyway whose average was a 3.7. My MCAT score was on par with their averages so my MCAT score didnt' compensate for my GPA.
Had I gone to the state school which this med school is semi-attached to, I was told by that school's premed advisor (who also wrote me a rec letter after I took her class), I would need at least a 3.7. But I could slide by with my 3.4 at my 'prestige school'. This is an extreme example since I also had other stuff going for me (tough major, work experience, took some more classes etc), but I've been told, had I gone to the local state school, they would have scrutinized my GPA a lot more closely.
My advice to anyone contemplating college is this: when choosing which school is right for you, consider where you stand in the applicant pool.
If you are the top or the bottom of the pool, I recommend attending elsewhere. If you are middle of the pool, stay there.
This is my reasoning: if you suck, no matter where you are, it will reflect badly on you. And when I say 'suck', I mean, getting C's or worse in your pre-reqs. Yes, you can attend med school with Cs on your record, but it'll be an uphill battle.
If you are top of the class, then you may not be fulfilling your full potential. Case in point: a 4.0er at State U may have hit the 'class ceiling', had she gone to Harvard, maybe she would have gotten a 4.0 there as well. And of course, the 4.0 at Harvard will carry more weight than at State U in admissions. So you'd maximize your chances of med school if attended a school which gives you maneuvering room on your GPA.
If you are the middle of the pack in your college, it's the right place to be. It allows you 'room to grow' but gives you enough berth to not see a huge drop in your grades. That's how I picked my school. Of course, getting alot of money also helped.
This advice is only solvant for future med/law/bschool students---anyone looking to attend grad school and who cares about their grades. If you only want to come out and work, attend the best school that you can
afford. Grades matter a bit, but they won't be scrutinized like they would by adcoms.
To the OP: you'll be fine as long as you are a strong candidate. Yes, a Harvard grad with a 3.3 will still probably make it to med school whereas you will need to work harder if you had that GPA but as long as you do well at your respectable college and do well on the MCAT, you will get in somewhere.
However, if you want to make it into a top med school, it would help if you went to a top undergrad. It won't guarantee anything, but from I've seen, top med schools accept a
disproportionate number of students from top undergrads.
At my school, only a small percentage of applicants (~10-15%) attended what I consider 'prestige' med schools while the rest of us went to the normal med schools...however, with a successful rate of 90% (from personal experience, I would concur with that), while the national average hovers at 50%, it's not a bad gig to attend a 'prestige' school. However, it's not going to end your med school career if you didn't.