I think GMAT gets in you in the conversation, nothing more, nothing less. In other words, a poor GMAT score can keep you out, but a strong GMAT score can't get you in.
The thread on Kellogg GMAT thread I posted a while back (not sure where it is) proves this to some extent - clearly there is a correlation between odds of admittance <640>700 - thats fairly clear. Once you are in the 700 range, whether were talking 720 or 730 or even for that matter 740, doesn't much matter - admissions411 proves that. I think there's a logical band at 700 to 750, and slightly better odds above that - but even then, a 750 vs a 780 doesn't appear to have any real meaning. I once ran a regression on admissions411 data - by combining entries for top schools - and basically found no correlation whatsoever between GMAT and admittance. The reason is - admission411 data is supremely skewed towards high GMAT scores - so in other words, in the over 700 range, it doesn't make a big difference.
Given that you can only control essay and interview, I'd suggest 95% of your effort on the essays, and 5% on your interviews. If you get an interview invite, you'll have time to prepare. If you focus on interview and your essays suffer as a result, you won't get the interview, and it's pointless effort.