I can at least answer your first question about the psi...6 psi for any stock engine is pretty much okay, some engines like the LS are rumored to be able to take upwards to 10-11 psi and still be *safe* though I sure wouldn't recommend it. I would check other threads on other forum sites and see what they've got to say about it, but 6 psi isn't too threatening.
Check out www.carreview.com to see if there's any info on Revhard turbos...I don't have time to check right now, but usually you can get at least an idea from that site.
I'd check out www.turbo-kits.com and www.turboneticsinc.com if you have any more questions about what turbos can do. I'm not trying to sell those products, but they are among the best, and they've got GREAT support (thank, Jesse @ turbo-kits.com!!!!).
If you do go more boost, by the way, look into new rods, pistons, sleeving the engine, etc...that was you can put a lot of boost in, though the GReddy itself is somewhat limited as far as upgrading is concerned.
revhard kit for rsx.....first one bought by a customer caught his engin bay on fire and burnt a whole lotta his engine. dunno if they've gotten any better now. problem was the turbo was placed way too close to the firewall.