First thing to do to get more HP to 99 Boxster?

[Luv2Box]

Steve is correct, there are mods you can make, depending on the state's air pollution regulations you live in, that can add minimal hp but when you calculate the $$$ per hp. mostly it is a waste on money. The only large hp gains I've seen, from the 2.5L, have been from turbocharging or supercharging, both expensive and controversial as to the potential damage they can do to the motor. You can always swap the motor to a 3.2 or 3.4L but then again it is expensive. The best advice, if you really crave more hp, is what another post recommended to sell it and buy an S and that way you get all the other brake and suspension upgrades as well.
You can also remap the ECU for another expensive for hp. gain. Typically around $800-$1200. I've known two people who did it and the hp gain was minimal and fuel mileage went way down.
I've been driving my 2.5L for six years and although I can't compete in a straight line put me on a winding, twisty road and I'll stay with just about anyone.