It is best to play the .hack//game first then play .hack//G.U. if you guys are interested in the story.
done that, read the novels of it, watched sign, watched bracelet of the twilight, read bracelet of the twilight and watched roots. along with reading all the little extra omake stories that come out with the G.U manga.
(and the other novels too once i get my hands on them)
we get everything BUT the next game series >_>