I do not think GRE is helpful to LSAT. Reading GRE articles, you need to pay more attention to the detail of the articles, while LSAT reading ask you to focus more on the big picture of the article. Most of the LSAT questions is reference questions or at least you can match it in the articles, comparatively, GRE asks more maching question which answer can be mirrored in the article. In a word, GRE is helful to the extent that it exposes you to English milieu.
agree with lawyer-1. their differences reflected in questions are more outstanding than differences in articles themselves though. if you have a whole lot of time before you take the test, to warm up with some GRE articles is not a bad idea.
It'd be helpful as well to read books on philosophy or political science and dense magazines &newspapers.