My votes for a three liter!!
Seriously, make sure you read into baffled oil pans and dual mode dampers as well for insurance on the car you're modifying.
Intake/exhaust is the best combination to start with.
If you want to get serious then start looking for SVT heads as the cam profile is good for more power. Also the SVT computer, throttle body and mass air flow (MAF) sensor are good starts.
After that look up chips and N2O and you'll be kickin' it.
Then, once you do that, look in the group buy section for my carbon fiber IMRC covers!!

(nothing wrong with a shameless plug)