It's about how you market yourself my man. I'm in an Ivy League school right now, and had a solid B average in HS. Hell, I've got a B average right now, but I've still secured two back to back jobs and grad school at another Ivy. You have to network, and figure out how to sell yourself as a package/product. I've gotten more out of my experiences on sports teams, clubs, and other organizations than I've ever gotten in the classroom.
So in short, no, grades are not the only thing that matter. You need to keep them moderately respectable, but most important thing is selling yourself, and how exactly you do that.
*EDIT*
Also, as far as frats/sorors/other things, I can only speak from my experiences... but they're massively beneficial. I've been interviewed by people in my organization (not greek) and talked about football the entire time. Been given job offers and everything. Similarly, at T-25 schools, the people in those frats have parents and "friends of the family" who are captain of industry. Aside from myself, I know a girl who interviewed with a soror and got an instant job offer with a Fortune 10 company. Another who didn't even interview and got an offer from Morgan Stanley.
You should, however, strive for a gpa > 3.5. I got a recommendation for Deloitte consulting and McKinsey, but my GPA didn't hit that 3.5 threshold and I got dinged. That McKinsey one really hurt BTW
. BUT... the network is what got me into that position in the first place. Don't overlook that.