I'm also a "low purchaser" compared to most on this board, but you'd be surprised how quickly your everyday purchases like groceries and gas add up.

Between the sign-up bonuses for new cards, 2-3 flights per year at a normal fare, and everyday purchases, I can easily meet my goal of getting a free flight for my yearly vacation. ^

As many others will tell you, the StarWood AmEx is a good idea. So it having a Visa/MC for places that don't take AmEx. Otherwise, just sign up for new cards when you see great offers... as long as you have the restraint to not carry a balance!