Question about *nix central authentication

• How easy is it to setup centralised authentication for a bunch of Linux servers (Redhat 4,5,6.1)?

• We are a large organisation and our window$ servers and desktop are part of a 2003/2008 AD domain, it's important (obviously) that if the answer is to use LDAP that this does not interfere with the AD domain.

• What the current preferred way of doing this (LDAP + Radius etc...?)

• What your idea of the 'best' tutorial / write-up for this?

Thanks in advance.