The preclinical years have only minor impact on most residency applications (pathology may be the exception). If you do awesome during these years it will help you, if you do terrible it will hurt. If you're average (like I was) it is not a factor either way. Doing well on Step I and your clinical grades in 3rd year (especially surgery, medicine, and the specialty you plan on entering) will get you a lot further.
I was pretty damn average in the preclinical years but Honored medicine, surgery, radiology, and a few others in third year (all honors in 4th year but these don't matter). I did pretty well on Step I but not stellar. I still ended up getting 25 interview offers for radiology and am now at a very well respected university progam in the Midwest. This program was very high on my list.
So, in summary, average preclinicals won't hurt. Do well on Step I. Do well in third year.