1. The most popular horsepower modifications to the Boxster are headers, air intake, ECU upgrade, and muffler producing 25 to 35HP gain for $5000 or so.
2. The other option is to swap your engine with a 911 powerplant (3.4L or 3.6L). This generally cost $12,000 to $15,000. A six-speed transmission is optimal(S version OK) and uprated suspension and brakes should be done in unison to accommodate for the extra speed/power of 300 to 340 HP.
3. Supercharging and Turbos can be done through a number of providers including:
Turbo Performance
http://www.turboperformance.com/porschef.htm
(great vid of boxster burnout)
TTP (Turbo Technik Pietz AG)
http://www.t-t-p.de/english/index.htm
(SELECT Tuning --> Porsche Conversion. Pricing included)
Gemballa
http://www.gemballausa.com/
Roock
http://www.roockusa.com/products/boxster.htm
There are a number of other tuners I have not mentioned. A great list of Boxster related tuners & sites is:
http://www.janesko.com/boxster/
Hope this helps.