I have a Celica with the 3sfe engine(5sfe). I have a chance to get the genie header for a good price. What kind of gains would I be looking at if I decide to go header, exhaust and cai?

I have heard people saying that I will lose some torque, is that true?.Just want to make it a little better to drive.

Whether or not you lose torque depends on the header. A header does free up some extra power... but it's not gonna be a huge deal where you can feel a significant difference.