While the game states that Int is the sorcerers primary statistic, this is not true. Int changes the amount of spells that a wizard can memorize per level. This is hard coded into the engine for sorcerers and therefor doesn't apply. The only reason for a sorcerer to have high Cha is for role playing purposes, personly my sorcerers always have 18 Cha in the BG series.
Powergamming wise, the best attribute to max out is wisdom. Some would say Dex or Str but if you need to use hand to hand combat with a sorcerer you did something wrong. That said, a backup plan never hurt. You're probably wondering why max out wisdom, one TOB spell allows you to rememorise spells in the thick of combat, this spell uses a wisdom check.
Hope that helps.
