PSI is a unit of pressure (pounds per square inch) and I believe refers to the amount of pressure that the forced induction method of your choice puts on your Intake
as for the safest pressure you can put on an unmodified engine, it totally depends on the engine and how much you want to risk. Any form of forced induction is going to put your engine at risk, and the higher the pressure the higher the risk.
A modest boost of 4-6 PSI is usually ok, especially on a GT, but even 6 on a GTS can be risky due to the GTS's higher compression ratio.
However, a friend of mine had a modified supra engine that he ran at ~28 PSI successfully (which he soon thereafter destroyed)
As for the gains, it again depends on the system and the mods already on the car... if you are using a stock exhaust then there is a limit to the amount of air that the engine can put through the system regardless of pressure.
as a point of refrence... the west corvina toyota site claims that their turbo kit will put out ~230 hp on a '00 celica GT w/ 6 PSI (from ~140)
hope that helped
PS- anyone feel free to correct anything they think is wrong... i'm not exectly an expert by any means