Like ghosthunter said, intake, headers and exhaust should be done together for you to feel a dramatic difference in the power. Budget maybe 20k or 25k for a full headers-midpipe-exhaust job at a good exhaust place... freemuff near evangelista is good... if you're in Laguna, gomags in Biņan is excellent.
Buy a glasspack exhaust (not the 2.5k cheapo ones) or a JASMA surplus for smooth sound.