How much grades matter depends on the institution you apply to. The larger the department or company, the more grades will matter, because they will likely be used as an initial screening criterion to "whittle down" the pool. A GPA below 3.0 is not going to help, and at some places, will earn you an instant rejection, work after graduation notwithstanding.
However, if the review is "holistic," and you have outstanding letters from your recommenders, then the impact of the GPA may be lessened. (It will not be completely overlooked, but at the least it won't be a criterion for "bouncing" you without further consideration.)
I should also point out that after graduating, depending upon who you work for, your GPA may still matter. At a start-up or small company, it may not be such a big deal, but I've worked for several large-scale employers who had rather strict GPA cutoffs (well above 3.0 for both undergraduate and graduate degrees!). If you were way under this level, it was just not in the hiring manager's interest to spend the time necessary to defend the hire, versus selecting a candidate with a better record. So you may be able to overcome your academic record, but you won't be able to bury it completely.