Starwood AMEX can help you accrue a lot of Starpoints, but it does not qualify you as Starwood Preferred Gold. AMEX Platinum (not the Starwood one) will qualify you, but I believe the annual fee is in the range of $350. Starwood has been known to be a bit liberal in handing out Gold for various promotions which has been a sticking point for those who earn it the old fashion way (10 stays or 25 nights per year). There are several threads on flyertalk.com debating the merits, or lack thereof, of SPG Gold.
As Gold, you are not entitled to the best available room on checkin, but a preferred room, which usually means high floor or SPG designated floor (same as regular floor with SPG member sign by elevator) and possibly a complimentary bottle of water. Gold benifits vary greatly by property. For a best available room upgrade you must be SPG Platinum.