Some of you guys need to understand what "higher" ratios means when talking about gearing. The greater the number, the lower the ratio, not the higher. Tall diffs have low numbers, such as 2.78 or 3.08 and lower ratio diffs have 3.73, 3.9 etc.
Fitting a 3.73 into a standard Ecotec/auto will simply mean faster accelleration, lower top speeds in each gear, greater engine noise and wear and, depending on driving habits, greater fuel consumption. Your speedo will need to be recalibrated or it will read approx 21% fast as Danja points out. You wouldn't need to "change the shift points" in the auto, as maximum speed in gears will be determined by engine revs.