Having read through this thread, I have a few comments. My perpective is as one who has done both basic science and clinical projects. Now, as tenured clinical faculty at a med school, I interview residency applicants every year.
First off, the fallacy of impact factor. Remember what impact factor is - it represents how many times papers in that journal are referenced. You can have very good journals in small subspecialities that have lower IFs than mediocre journals in large subspecialities. Rely on you PIs advice as to where to submit. Talk to people in your prospective field and you will quickly get a sense of which journals are the most respected.
Second, quantity vs quality. When I interview residency applicants, I look for several things - where on the author list is the applicant, how many publications are there, and what journals are they in. I would rather see fewer first author pubs than many mid-author list pubs. I would rather see fewer high quality papers than many crummy papers. However, it is ok to have a mix. I look at the pubs list and generate an overall gestalt. One caveat though, if you have a publication (or several), be able to discuss them with your interviewer. Not knowing about the work, even if you are low on the author list, is a big no-no.
Third, the chance of getting a first author basic science publication without taking one or more years off is essentially zero. It does happen, but mostly with luck in landing the right PI and the right project. If you are not going to take a year off, feel feee to spend your summer doing basic science, but plan on getting in the middle of the author list, if you get anything. Without a year off, clinical papers are much higher yield (chart reviews, databases, case series/reports, etc).