Oh my god... I found a 17 year-old who didn't do stereo mods before all else =P
I'd go for a nice headunit up front and replacement speakers. That should come out to just about $500 exactly... $150 for a set of 6 1/2" components up front, $150 for a set of 6 1/2" speakers in the rear, and $200 for the unit itself.
If you want performance for around $500, you could look at a TRD
exhaust. It's logical since you already have the CAI