Cheap mods:
9005 conversion (makes night driving 100x better)
Copper spark plugs
Advance your timing 2-3 degrees
LT's suggestion for the trunk tie bar

Cheaper mods:
Comptech Rear tie/sway bar
High flow cat
Comptech, DC, or GReddy header (4-1 if you play more in the high end, 4-2-1 if you're more of a street driver)
Skunk2 intake manifold
Catback exhaust

Expensive but good mods:
Tein SS (street) or Omnipower (more street/track) coilovers
Cams (ITR or Skunk2 stage 1)
Air/fuel management