I disagree with two notions here:
1. An improvement from a 730 to 770+ is slightly more difficult than an improvement from 640 to 680. I've come to the realization that it's not much more difficult. Yes, going from 640 to 680 is easy because at that level, the questions are much easier. But if you can reach 730 consistently, you can get to 770+ as well by taking more care and by getting lucky with some questions.
2. Doing worse on your second gmat than your first might be a HUGE negative especially when your first attempt was an excellent score. However, if your score is the same or better, bschools will look at the higher score and give you credit for the higher score. Afterall, they want to boost their stats.
You also have to realize that there are many people who score 700+ With that many people scoring so high, a 40 point difference won't make much of a difference in the eyes of adcoms. If you weren't going to get in with a 750, you're not getting in cause you have a 790. Think about it, do you think adcoms are impressed by an 800? Probably, for about 5 seconds.
But I do empathize for those who want a higher score for personal satisfaction. It is this drive that made us prepare furiously and score so well in the first place! But is it really personal satisfaction? Or is it so that you can boast of a higher score when someone asks you about your score? Well, if and when you do tell others of your new score, remember to tell them that you took it more than once as well.