Bang for Buck Mod

[BoxsterSteve]

The best bang for the buck is an underdrive pulley. Count on 5-7 ponies for about $200. Cat delete pipes can also net you 5 or so hp for $200.
ECU tunes are really expen$ive, and may net you 15 or 20 hp if lucky.
Spend the other $700 or so fixing the other rear fog light...

[tommy583]

I second the under pulley mod. Desnorkle is a good free mod. It may not give you any HP but it sounds nice.