Mustang gears! need info!!!?

I GOT A 1994 MUSTANG GT 5.O IM GUESSING MY REAREND IS 8.8 I WAS JUST WONDERING WAT GEARS WULD BE BEST TO PUT ON IT....I WAS THINKING SUMTHING LIKE 3.73 OR 3.90 I WANA KNOW WAT WULD THE 3.73'S DO AND THE 3.90'S AS WELL...AND I ALSO HAVE OVERDRIVE ON ON MY CAR SO WAT DIFFERENCE WULD IT MAKE? AND IM PLANNING TO PUT SOME 20'' RIMS HOW LOW WULD EACH BE DROPPED TO ? AND WAT WULD I NEED TO CHANGE IF I PUT 3.73 OR 3.90