This is titled a "modification and tuning" guide, but is a great buyer's guide.
997.1:
http://sharkwerks.com/porsche/porsch...07-2008-a.html
997.2:
http://sharkwerks.com/porsche/porsch...10-2011-a.html
GT3 is the only direction I can see myself going post-M3. Don't care for the diff, RMS and coolant pipe issues though. It has a reputation as being particularly stout once sorted, but some of these fundamental issues are pretty ridiculous given the purpose of the car.
Might just be worth it though. My ride in a 4.0 on track was, memorable.