Are 3.73 gears a good gear ratio?

2000 trans am ws6 auto with slp lid flowmaster exhaust and rebuilt tranny changed shift points. nitto 555r drag radials. the only bad thing is it has 153,xxx miles but a mechanic/use to be drag racer said miles wont efect car much. im putting 373 gears instead of stock 323 and im getting a progammer and change the shifting from 6000 rpm to 6500 rpm and tuning it for racing with the programmer and possibly headers. how much horsepower will i be running then? just a good guess plz