I believe the 04 GT3 come with the Billsteins PSS9 kit for the 996.
You can get the 986 kit for your Boxster for about $2500.
The ability to adjust the ride height and rebound is tremendous.
Just get out the impact wrench and race jack and you can raise the ride
height back to stock height in less than an hour.
http://mindovermotorsports.zoovy.com/product/GM58877HO