Hello Roshh,
->You can take the test when you are fully prepared. It will take three to six months of intensive study depending on your capacity of learning, proficiency in English and analytic ability.
-> Yes , old pattern of GRE is going to be changed soon- from 1st August 2011. It will be much more difficult than the present style, for example- you will be allowed to use calculator in the upcoming pattern which indicates that the analytic section will be very tough.
-> NOPE, GRE is a must to do to get an admission.
-> BARRON"S GRE is the mother book for novice GRE students. NOVA math bible is the best for quantitative section. Other books you can use are-Kaplan math workbook, Word Smart for Vocabulary and BIG BOOK- where you will encounter 27 real GRE tests. You must buy some GRE softwares for mock tests before the real exam.
-> You should register when you are totally prepared- three weeks to one month before your desired test date.
->
http://www.ets.org/gre
Hope this helps. Thanks.