Don't do lowering springs unless you plan on doing shocks. Save up a little more and do springs/struts at the same time. Unless you want to just upgrade the struts only.
Personally, I'd suspsension first as you will feel/enjoy that more then simple bolt on power. Do exhaust if you want more sound.
Or you could do MAF adapter/K&N, replace resonator and get the y-pipe optimized for less then $400. Then if you still don't like the sound save up for mufflers next.
