The best bang for you buck is going to be a tune. I recommend a XCal from FSWerks. It'll run you just shy of $400 and you'll get more out of it than intake or exhaust. But to be painfully honest with you there isn't much you can do to get that spi to get up and go. Either focus on handling or sell it and get a Zetec or a Duratec.
understeering? visit FealSuspension.com
for quality repairs and modifications in the LA area: StevesAutoClinic.com