Since you have a 99 you can get a set of headers with no cats from Fabspeed for $1.1k. Ideally you want to get the catback as well since it'll remove any restriction in the exhaust.
http://www.fabspeed.com/Boxster.html
Underdrive pulley is also an affordable and effective mod.
It is a slippery slope with power. Might as well learn how to drive it properly and spend the money on stickier tires.