The two parts from the 996 GT3 that bolt right on to the 986 are the GT3 front sway bar and the GT3 adjustable control arms (allows you to get more negative camber without excessive lowering; good for the front suspension, optional for the rear). You will need to get different drop links for the GT3 front sway bar (Tarrett et al). Rear bar size will depend upon how much tire size stagger you have front to rear, how much power you're putting down, whether you've put a LSD in or not, and how loose you like the car (so start collecting rear sway bars). The lowest price I found on Porsche OEM stuff is at Sunset Porsche in OR.
You probably know the drill on camber (keep on increasing the front into the -2 to -3 degree range, depending upon what tire you're running; probably leave the rear at around -2 ish). A good starting point is 0 toe front, 1/8" toe-in rear.
I wouldn't go too crazy lowering the car, because 1) if you lower a car with a Mac/Chapman strut suspension front and rear too much you will loose camber reduction on the INSIDE tires during a corner due to chassis roll (and just grind the inner edge of your inside tires) (so you want your lower control arms at least parallel to the ground if possible), 2) you will get both front and rear geometries into ranges where bump steer is more pronounced, and 3) with excessive lowering of the rear, you will run out of toe adjustment. (Not everyone will share this view; there are plenty of pricey aftermarket suspension links to mitigate 2) and 3)).
When you get the car's suspension setup, and hit the track on R-compounds, you will be amazed at how quickly a mid-engined car can change direction without getting unsettled...
The tarett front bar is the stock GT3 front bar, get the long drop links from tarett they give better geometry and don't bind like the short ones do. I have a 4 inch crank pulley and a accusump adapter for your engine if you are interested shoot me a message.
+1 on the underdrive pulley. Boxsters and Caymans were not really designed with sustained high(er) revs in mind, so a lot of sustained revs in long corners tends to increase the power steering temps (which sometimes leads to a melted P/S tank) and accelerates the wear on the alternator's brushes and the water pump seals. The underdrive pulley addresses all of these issues, and gives back a few HP as well. (Make sure to buy a slightly shorter serpentine belt when installing an underdrive pulley.)
For just the P/S, some have installed coolers, and some have even switched to electric power steering pumps.
Depends on class rules, but my 'ideal' suspension set-up for these cars looks like this:
Shocks: Moton's (Clubsport's for me, but if you're 'big money' the Motorsport's are pretty awesome).
Front end: GT3 Control arms, GT3 Sway bar, Modified sway bar links, Tarett camber plates, Tarett bump-steer adjustable outer tie rod ends
Rear end: GT3 Control arms, H&R Sway bar, GT3 cup toe links, Tarett locking plates for factory toe adjuster eccentric, WEVO engine / transmission mounts, Mode upper monoball shock mounts, and I'd like to make a sturdier brace to connect the two side 'horseshoe's' for the rear suspension carrier's.
There are a tonne of extra parts you can throw at these (monoball control arm bushing replacements, solid caster bushings, thrust arms with monoballs, etc), but I haven't yet seen a need for them in my applications.
It IS all about the rules though....unless you're just out for lapping.
All those parts can be had from any knowledgeable shop that handles a few of the required product lines (Moton, Mode, Tarett, WEVO, Porsche Motorsport, Porsche OEM). Also, if you look at many of the supplier's web sites, you'll see their list of dealers somewhere on the site.