G.I. Joe has been the Real American Hero for over twenty years, and is now jumping into the 21st Century with both feet forwards.      Following up on the incredibly successful 3 3/4" action figure toyline and multi-media extravegansa, Hasbro and Gonzo Studios are joining forces to present a G.I. Joe for the kids of today.  With a decided stylistic flair and an emphasis on secret agent technology, Sigma 6 will fight COBRA: The Enemy starting on September 10th. Showing in half hour blocks, the producers promise a solid continuity throughout the episodes, starting off with the ever-popular five-part miniseries much as the animated series of the 80's used to do.