If I want to sit for the GRE, what is the best way to prepare for it? Need I take the IELTS or TOEFL?

GRE and IELTS/TOEFL are not relative to each other. While GRE checks for your quantitative and verbal /nonverbal ability, IELTS/TOEFL seeks to know your command over the English Language. For the start you can pick up GRE Barrons..and enroute your preparation you will find many guiding post. IELTS/Toefl : based on the country/University you are aiming to get into , you have ot select either of the two ..and then get onto it. IELTS : http://www.britishcouncil.in/exam/ielts;  TOEFL : http://www.ets.org/toefl?WT.ac=toeflhome_why_121127 BEST of LUCK :))

I think preparing for TOEFL will not really help with the GRE. The pattern and objective of both the tests is different. The GRE tests your vocabulary and ability to interpret passages and answer questions. These questions usually do not have very direct answers and you should be able to infer or extract the correct answers from the passage. The GRE also has a quantitative section covering a variety of topics. You could use Barron's vocabulary list and  the official ETS GRE guide for the major part of your prep and books like Manhattan 5lb, etc for the sheer number of questions they have. ETS also provides 2 mock tests called Powerprep which are similar to the actual test. You could give the first one after a few weeks or months of prep (depending on the time you have) and note down all the problem areas in both sections. Then you can give the  second test about a week before the actual exam, so that you have time for corrections and a final revision. Ensure that you take these tests under conditions similar to what you will face on the final test day. The main issue I had while giving the GRE was the nervousness and exhaustion towards the end of the exam. So be well prepared and stay calm throughout. TOEFL on the other hand tests your reading,speaking,listening  and written skills. I think the reading comprehension questions are a little more straightforward. You can use the TOEFL testing software that ets provides to prepare for the test. I hope this helps :). Remember that these are only suggestions I could come up with from my preparation for the test and you don't have to do it the same way.