My list for you:
1. 180 T-stat. Your engine is prone to Lower Intake Manifold (LIM) gasket failure which can destroy your motor if not addressed. The t-stat will be a good preventative measure.
2. Intake: Fenderwell intakes seem to be the most popular mods around here.
3. PCM: Small gains but it makes most other mods you do more affective. Intense has a page that gives you advice as to what tweeks would be best for you.
4. Shift kit: Tranny failures do happen, more so on supercharged 3800s but it still wouldn't hurt. Plus it firms your shifts for better acceleration.
5. Rockers or headers: You'd have to pick only one to stay under750 and they both have different gain characteristics.
Exhaust mods other than headers won't do much for power but a new muffler and installation will probably run under 150. But thats if you don't want tips too.