if going NA you wont see any gains going from 2.5 to 3. if going turbo then go 3" regardless of when you actually get the turbo set up. just get as big of a resonator as possible for the straight section.
2001 ZX3 2.0L Zetec-
Hatch Nation #42
Going through Rebuild See "The TICK Build Thread" for details (http://www.focusfanatics.com/forum/showthread.php?t=289168&referrerid=62897)