Looks like you've got very good advice from others, but just thought I'd add my experience in case it is useful. I recently returned from the US and made good use of the Amex Global Transfer - initially applied for the SPG Personal Amex, then Platinum card, Business Gold card, SPG Business, Hilton and Mercedes Benz Platinum cards over the course of ~8 months. I wasn't eligible for a SSN but got an ITIN by opening an interest-bearing bank account.
The first application had to be manually approved (applied online, rejected, then called to have it reviewed on the basis of Global Transfer), but the rest were instantly approved online (with ~2 month intervals between applications). I wasn't worried about building up a US credit history, just taking advantage of sign-up bonuses while I could.
The main benefit of the Platinum charge card for me (other than the signup bonus) was airline lounge access, and the airline fee rebate ($200 per calendar year; can be used twice in your first card membership year). The Gold cards have bonus point categories for earning Amex MR points.
I put most of my spending through the SPG cards with a view to ultimately transferring those points to AA for Oneworld redemptions. Where there were bonus Amex MR points available I'd spend on the Gold card - haven't used any of those points yet, but probably will transfer to BA for Qantas domestic J redemptions or to SQ. Signup bonuses for the charge cards can be 50-75,000 points so although Amex MR may be less valuable than SPG, you can accumulate them more quickly.
Good luck, and enjoy a whole new world of points earning opportunities!
Mike
