For ISB the 80% GMAT score distribution is 680-760 with average being 716. At the sametime ISB has admitted applicants with score as low as 600.
The problem is usually with the most typical pool - IT Indian Males. If you are from this pool, you surely should target around 720. A good gmat score will keep you competitive
. Otherwise anything around 700 (+-20) is pretty good.
Having said that, ISB loves adding diversification. So if you are from a job background which is quite unique, a lower score would also work. With the current score of 640, you'll need to have something extra-ordinary in your application and must have very strong essays.
Also you may be interested in the below threads related to ISB:please-join-me-in-welcoming-thinkisb-to-gmat-club-84103.htmlindian-school-of-business-2010-11-thread-82576.html
ISB 2011-12 thread | Ask ISB Alumni @ ThinkISB
All information related to Indian candidates and B-schools | Indian B-schools accepting GMAT scores
Self evaluation for Why MBA?