Have you heard about HBS 2+2 program or others such as Kellogg's MMM program? You may want to look into those since they cater specifically to the juniors/seniors in college. Stanford has a similar program as well I believe.
1. I have seen a few folks go ahead and take the test while they were juniors/seniors in college. There is some kudos for that as you show a pattern of long-term planning, and discipline. it is also much easier to get a good result while your mind is still quick, your memory is good, and you have ample time for studying.
2. At the same time, the GMAT is only good for 5 years and frankly after about 3 or so years and definitley after 4, it is considered stale (e.g. you may get 770 now but the admissions team when reviewing your application, will consider it not as a strong factor as for someone who is working a 60-hour job and was able to get a 740). They are people (smart ones usually) and understand that the responsibilities of a senior in college and a lead consultant with 2 kids are very different, therefore output is to be expected to be different
3. Numbers are not as critical as the interview, letters of recommendation, and frankly your personality. Essays can be faked, and experiences inflated, but you as a personality will jump out during the interview and during other aspects of application and that's been much more important these days. That of course relates to the top 20 schools mostly.
In short, I would say that if you are not applying to the HBS2+2 or other such programs, it is probably a bit too early to take the GMAT (you are spending a few hundred on the test and a few more hundred on the books).... maybe or many not be a great investment since you may not even head to the bschool. At the same time, if you have the time and the money, and certain you will head to the top 20 schools, then by all means, take the GMAT, if anything it may prepare you to be better suited to retake it later (even if you miss the 5-year expiration deadline).
The better approach is the
MGMAT books or the Veritas Prep Books that provide a much more detailed overview.
You also may want to download the GMAT Prep and take a practice test to see where you stand (you may be up there already). Others also offer free diagnostic/practice tests that you can rely on, including Kaplan.
Good Luck!
BB