How many miles do you have? Do leakdown and compression tests to check the status of your engine. If its healthy i'd run 8 psi, you could run 10. The more you run the harder it is on the motor. If you get a block guard you could run 12psi. Thats what i'd do if you dont want to do a full build.
Well, new stuff. New, Better stuff. Stronger bottom-end. You could get a BC stroker kit for the B18 and be set, and have a bigger displacement to boot as well.
If you're scared of running high boost, i've got a turbokit for sale that's wastegate is set @ only 5.7psi. And it's only $800... Even with just 5.7psi you should be looking @ atleast an 40-60whp gain.
But for sure, check out your car's maintenence stuff before doing anything major. Check for vacuum leaks, coolant, oil leaks, check your spark plugs, maybe even get new ones, etc.
ok, do a comp. test, see if your motor is healthy, make sure your not leaking oil or coolant, or burning anything. if your motor is healthy your okay to boost, and if you want to build your motor, then build your head that where all cars make there power, but if you want to be able to run more psi, then build up your bottum end, dont just get block guard, they blow, go read about them, get the motor sleeved and get stronger rods, and low comp piston, there is alot of info on here about building motors for boost... do a little bit of research before you get in over your head