Something to buy you more time on the gaskets and get a bit more performance out of the car in warm weather is replacing the stock 195* F thermostat with a 180* stat. Heat and pressure make the gaskets fail, so reducing the heat is good. With the stock 195* stat coolant temperatures around 200* and above are common. The stock computer trips the fans at 212*, which is easy to see in stop-and-go city traffic during the summer. Also, running the A/C automatically trips the fans regardless of coolant temperature.
I've been running a 180* thermostat in my 99 LeSabre for a year now with no ill effects. The heat in the winter is enough to roast a chicken, and the motor warms up quickly.
Another place for a slow coolant leak that will drain the overflow tank in a month and result in poor gas mileage is a bad radiator cap. Mine went on me around the 9 year old mark, and I could barely get 25 mpg highway. A new radiator cap cost $5, and a system pressure test to rule out gaskets was $50.