Anyone facing these questions should check out Steve Hsu's blog. Here are a few relevant posts:
http://infoproc.blogspot.com/2005/02/tale-of-two-geeks.html
http://infoproc.blogspot.com/2006/03/success-vs-ability.html
I will say, Steve's opinions are a bit pessimistic when it comes to this question.
If you are fortunate enough to get to study String Theory at a major university, and you complete your PhD, this says something very important about you to employers. Of course, this can be proved other ways, as wel, without the huge opportunity cost.
Opportunity cost is a good way to think about it: your degree may end up costing you on the order of a million dollars, not out of pocket, but in future earnings. The question is, is it worth a million dollars to have a PhD in physics? Steve Hsu would say probably not, I would say I wouldn't sell mine for ten times that :)
As far as affirmative action, I don't know. I mean, it definitely can't hurt for you to be (say) African-American when looking for a post doc, as physicists themselves tend to be more liberal. At the end of the day, though, you still have to do good work. The best people don't always get the best jobs, which is a fact of life, and as much as physicists like to think they live in an egalitarian meritocracy, this is categorically NOT the case.
Race may or may not be an ultimate factor in getting a post-doc, but it probably will be a factor if you apply to grad school. What I've noticed is that most students at the top programs and students at the second tier programs aren't that different intelligence-wise. (There are specific exceptions, of course. I only want to make a broad statement.) What you will see is many more women and minorities in top tier programs---this has to be due to some sort of affirmative action, as the quality of students (in my opinion) is not that drastically different. In other words, if you are a good student, but not a great student, and you're a woman, you are more likely to end up going to a better grad program. This has been my experience, at least. And realize that race or sex is certainly not a free ticket to a PhD---you still have to be better than 99% of all of the other undergrads applying for grad school.
I want to be very clear on this point, so I will restate it again: there are a pool of qualified applicants to physics graduate programs, whose qualifications carry some distribution. Some are more qualified than others, in terms of GPA, GRE scores, and undergraduate research. It has been my experience that the race or sex of the applicant can have some bearing on the admission to a specific program. Note very carefully that I haven't expressed an opinion on this subject, and I've only tried to relate my experiences :) So take it as you will.
Finally, to end the politically controversial section, I'd say you should always get the highest pedigree that you can, in the chance that you leave physics and do something else. For example, I go to a (high) second tier grad school, but my advisor is a world expert in his field---he essentially invented it. I chose to come here specifically to work with him. Irvine is the same type of program---high second tier, low first tier. For example, I think Jon Feng is there. He is a world class guy, and you may meet him and find you have a great rapport with him. And if you stayed in physics, he would get you in the proper circles. But if you want to leave physics, you may be handicapped by the fact that you are coming from a place that isn't a top 5 program.
So if you have the choice between CalTech and UCI, and there's even a CHANCE that you might leave physics, you should strongly consider CalTech.