most stock cars can handle up to around 10 PSI with out internal damages assuming everything is healthy. I would do a few tests (compression test, oil analysis) and go from there. I would start out at 5 PSI and go up from there after getting use to everything. There is a Turbo/Forced Induction Section which if you have time to browse will help loads in your quest for more power.
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)