SPG = Starwood Preferred Guest. It is the loyalty program of Starwood Hotels and Resorts. American Express has a credit card that earns 1 Starpoint for each $ spent on the card. These Starpoints can be used at Starwood hotels or they can be transferred at a 1:1 ratio to most airlines. Above posts have mentioned a bonus when transferring points to miles. If you transfer 20,000 Starpoints to an airline, Starwood will give you 5,000 more points, for a total of 25,000 points. Hope this helps.

To the OP, depending on your spending and/or flying habits, you may want to look at AMEX's new Delta credit card, the SkyPoints Credit Card that earns SkyPoints that can be used as discounts on any Delta Coded flight purchased through Delta Airlines. That may be another option. Personally, I would go with the SPG Amex because the fee is much less and, as others have said, the points have a greater value.