the front is already a 20mm. the rear is only 17. upgrading the rear will help your turn-in. the front you would have to get an aftermarket one, and unless went 22+ probably wouldn't notice. don't bother with strut tower bars except maybe in the hatch. the problem is it interferes with the great storage, and add only a marginal improvement. you can upgrade endlinks too, but i didn't notice that upgrade half as much as the rear sway bar.
('02 obs wagon)
you can upgrade everything, but you wouldn't notice the change much unless doing autocross-like activities and they will definitely increase the suspension noise and stiffness of your ride during daily driving.