From what I've read here on GC, yes, Ole Miss rush is very competitive. Makes me glad I went to school in the nice laid-back Northeast
Be sure to line up some letters of reference. Maybe one or two of your DG friends can write something for DG? (depending on DG's reference policy - some sororities require that these letters come from alums, but even if that's the case, having a letter from a collegian in addition probably won't hurt)
And keep your options open. There are a lot of great groups at Ole Miss.
Good luck! And be sure to start a rush thread - we're all addicted here