[Lucky]

If your Boxster is a '99, then 996 LSDs won't work. The base model boxster (that is, non-S) has an Audi-sourced 5-speed tranny. The 2000+ S model shares its 6-speed tranny with the 996, so 996 LSDs do work in it.

Many other Boxster parts are shared with the 996, for example the front suspension is nearly identical, so you can put 996 parts (including GT3) up front. Rear suspension is unique to the Boxster, so you'll have to source Boxster-specific upgrades for the rear.

Depending on your goal (DE, PCA club racing, NASA GTx class, SCCA, etc.) there is a huge range of choices you need to make, or in some cases aren't allowed to make. For example, PCA club racing is very restrictive if you wish to remain in the stock class. And jumping to GT classes can get very complex and expensive if you want to be competitive.

Give a little more detail on what organization and class you plan to race in and we can give you more specific input.

Good luck.