A math wiz a year older than me got an 800 on Math (and mid-600's in verbal) and a 5 on the AP Calc AB test and got into Cornell with that. He was ranked 14th out of 200 or so (not Cornell material usually) and was so piss poor in extracurriculars that he couldn't even make the National Honor Society. But his math testing ability got him into Cornell, so I'd say that great test scores do help immensely.
As for me, 700 Math/730 Verbal and a 5 on the AP U.S. History test and I got into Georgia Tech (where I will go), Vanderbilt, and Washington U. of St. Louis (great, but not very known university). BUT, I was rejected to Cornell, Rice, and Northwestern. That might be because I didn't bother to interview with any of them, but oh well. Tech was my first choice, I got accepted before I even started to fill out most of my apps, and I should have quit right there, but I probably would have gone to Cornell if they accepted me...