Reading Macintosh

I need to reinstall and reconfigure six PowerMacs 4400 and configure them to enable surfing and maybe IRC. I have already tried many things, but these machines are way too slow for newer stuff, could someone propose what software to install so it runs nicely and pages render as they should (support for Flash or Java is optional, but wanted).
And the second question is how to diagnose a mac, what is broken, some of them have maybe some RAM problems, others have HDD problems, so I would like to have some fairly correct method to discover them.