I'd say the EC tune is probably going to be the biggest kick for a single upgrade probably be running a sale on it soon with the new software!

Throttle body upgrade to 74MM will add a little more pep, and then you can upgrade the intake system as you see fit. (I'm just running K&N filters and noticed a nice little increase with Throttle body and descreening-debaffleing of the MAF housing and elbow)

EC tune around $350
Throttle body and larger MAF housing around $140 top of scale (patiently waited and got mine around $75 total)