The engine's been basically the same for '02-'05, except for ECU changes. '06 got a larger displacement engine (2.5 liter).
The question about how much boost the stock engine can hold is more a function of "How long do you want the engine to last?". You can put a huge honkin' turbocharger on there, and run 20psi, (with supporting mods), but sooner or later the motor's gonna go if you push it. Any mod worth doing will put more stress on the engine and shorten its life span.
I wouldn't go much higher than 14-15psi on a stock turbo; you're getting out of it's efficiency range after that point.
I don't think there are forged parts at all in the stock WRX engine. Can't remember for sure about the crank.
All of this info is available with a quick search, btw.