I can not remmember the visa classifications (hb-1, b-2, k-1, etc,) but I believe that anyone with a visa other than a simple tourist visa can apply preaty much immediately for a permanent resident alien visa (green card).
Well you can change your visa status from b2 to any visa status. i am on h1 visa and due to layoff i change my status to F1 visa. and a person who helped me is Ren Parikh. he is a director of vision career consultancy. you can call 732 283 2666. trust me, he is the cooolest guy. you should atleast call him for advice. its free. i have my persopnal experiemce and he is the best.