Nitrous is the cheapest power adder you can get, bottom line.
From what ive read, 75hp wet shot is probably the highest you can get safely with everything else stock.
Zex makes a nice nitrous kit.
2002 Sonic Blue v6 5-speed
Mods: K&N FIPK2 CAI, Motoblue 42% UDP, Steeda Tri-Ax, next up - time for a new clutch I think and a lightweight flywheel if I can find a relatively inexpensive one...