theres a decent chance you can get the AMEX charge in the next few months. I know amex was sending me pre-approvals for green and gold @ 7 months history. I'd wait to see if they offer you something before apping though. Discover and FNBO also sent me pre-approvals around 6-7 months. so wait a month or so after you hit 6 months and see if anything comes up first.
