Ye' old general discussion board. Basically, for everything that isn't covered elsewhere. Come here to shoot the breeze, shoot your mouth off, or whatever suits your fancy. This forum is not for asking programming related questions.
If your using shared hosting make sure you do some testing first. So far I've had only two of my hosts offer PHP5 neither of which have per directory PHP configuration working with PHP5. I'm being to think I'll be stuck with PHP4 till hell freezes over.
If you have control over your server, why not install PHP5 and run that on the same server. If you like it, go with it. If not, or if things go haywire, you still have PHP4 as a fallback and can easily switch back without changing anything related to your PHP4 install.