Cumulative GPA is more important, but having a high GPA in your major could help a bit, probably more so if your major relates to the area you are applying to. If you kick butt on the ASTB, get involved in volunteer work and such around campus, get great Letters of Recommendation, and do everything else that needs to be done to put together the best possible application package then you could definitely offset a subpar GPA. I've seen quite a few people around the forums who've gotten in with 2.8ish GPA's. The whole person concept isn't a myth. If its what you really want then go for it and don't get discouraged.
I imagine its more competitive after graduation because then you're competing with more applicants (other recent civilian grads, enlisted applicants, and people who have been working in the private sector for awhile). I agree with Version2point0 - might as well go for BDCP and if you don't get it then apply again for OCS after graduation. Be persistent and good luck.