Well, like others have said, first thing is to flush out the Dexcool and use green coolant. #2 Change spark plugs #3 New front O2 sensor
Also consider, as TurboTrav mentioned, putting a can of SeaFoam in your gas. SeaFoam it is awesome stuff and best used as a preventative measure. I highly recommend doing this as step #1.5
And an oil change very 5K, as always.