How much research experience do US grad schools really expect from an undergraduate CS student?
The general answer to your question is, there are no expectations per se. Although your question is phrased in terms of undergraduate research, your explanation focuses mostly on visibility in the form of publications. By parts and based on my own experience:


Undergraduate research: This is highly desirable, importantly because it's the best way to get an in-depth letter of recommendation. Letters are the most underestimated factor in admissions by students. The difference between a letter from someone we know describing at length your qualifications and a letter essentially re-stating that you got an A+ in a large class is very significant. Another reason why undergraduate research may be important is because it may give your statement of purpose more focus. Last but not least, it's always desirable to get a student that knows what he/she is getting into and that can hit the ground running.
Publications: If you have a publication in a top-tier venue, that is of course an excellent thing to have. A publication in a low-tier venue won't count for much by itself; its significance will depend on, you guessed it, what your adviser's letter says about your contribution.


Ultimately, I think the questions you want answered are:


Will someone with undergraduate research experience and a very strong package (including letters of recommendation), but no publications, be competitive in CS? My answer is Yes.
Could such an applicant lose over someone identical but with some low-tier publications? My answer is, these two applicants would probably be considered essentially equally.
Alternatively, could such an applicant lose over someone identical but with one or more publications in top-tier venues? My answer is Yes, IF they are competing for a single spot--which is unlikely. Also, such kind of applicants are not at all the norm in my experience (which could vary by area).


In any case, it probably can't hurt for you to engage in research that aims at publishing at a top-tier venue: When you shoot for the moon, even if you miss it, you often end up among stars.