Hi vas,
I wouldn't bother with the initial Star Trek movie releases as they are not the best quality. As William mentioned, all of the films are being re-issued to provide "marginally" better transfers.
The bulk of the earlier Trek films are not the most spectacular film stocks in the first place. I bought the newer 2 disc Star Trek The Motion Picture directors edition and was really disappointed. The trasnfer is packed with artifacts that could stand to be significantly reduced. Most of these films look very soft to begin with and there is not much you can do about it. It is not until you get to First Contact that you end up with a good quality disc.
If you buy the old set you get the original releases, but not the greatest quality. Wait for the new discs if you don't mind revised editions, or if you enjoy all the extra features.
All that said, the directors edition of The Wrath of Khan will be released on August 6th, and I am looking forward to it. The third film should be out before the end of the year.
Howie