if ur geting 15" wheels go for 16v as there more common than nova gsi and better, also more practical than v6 brakes.
the servo is the big disc looking item just behind where ya brake fluid goes, you will need 1 off a 'NOVA' GSI/GTE/SRi/TD/D.
by upgrading this you get more assistance off the car when you brake so less pressure on pedal. if you get 1 of another car such as astra/cav it wont fit unless u get the end changed over (welded).
to change this you undo the 2 nuts holding it to the cylinder, 4 bolts holding it to the car, and unclip the connector from under the passenger side of the dashboard, probly 30mins cuz the 4 nuts r fidly.