SRT8 air box if it doesn't already have one and a tuner are the easiest and give good gains for the money, headers and off road mid pipe for the most power.

I put JBA shorties and catless mids on my wifes '06 Charger and picked up 4/10's, it was running 14.30's stock, the SRT8 airbox and Diablo tuner got it in the 13.80's, headers and mids knocked it to 13.40's, a catback didn't gain anything but did make it sound better. This car is really heavy too, probably 4400# I could probably strip it out like most guys do and run 13.20's.