Everyone else previously has spoken the truth: you have to have some level of decent, acceptable grades and board scores to get interviews at reputable programs. However, I will say with 100% CONVICTION from personal experience that your activities can certainly put you over the edge and get you interviews that you would not have otherwise have received.
Certainly - without a doubt - the biggest bang for your buck when applying is your USMLE Step 1 score, followed closely by your clinical grades. After that, though, having some significant, meaningful leadership experiences can definitely enhance your resume and translate into a better application.
When I applied, I had a slightly above average Step I. I fretted about this for months. I figured I didn't have a snowball's chance in getting a "prestigious" residency. I had, however, spent my first and second years in medical school being significantly involved with a lot of key activities (leadership in AMA, chaired my schools board of directors for the free clinic, nationally involved in the AAMC, etc., etc.).
Once it came time to apply to residencies, I got almost every single interview I applied for (save one) and matched at my top choice to a program that (I believe) will be a fantastic fit for me. Would I have got there on my scores/grades alone? Never in a million years. In my case, it was the "total package" of decent (but not stellar) board scores, above-average clinical grades, and then some great extracurriculars that carried the day.
The main caveat here is that you actually have to do something meaningful. I posted this previously, but (in my opinion) to have an extracurricular carry some weight you need to have the following components:
1) Show an ongoing, dedicated interest in the activity (not just one year of school)
2) Have actually done something (i.e. organized an activity, made a difference - not just attended meetings and paid dues)
3) Hold a leadership position of some sort (this doesn't have to be an elected spot, just take the initiative to do something - this goes along with point #2)
Like I said previously, everyone here is correct in saying that your scores and grades generally go much further than your extracurricular persuits, however I would add that having significant extracurricular experiences can lower the "threshold" for how good your grades/scores need to be to get a second-look from the PDs when applying for residency.