You can get a Samba server to authenticate off a Windows PDC or a Windows AD server. Samba can also work as a Windows PDC. The next version of Samba will be also able to function as a Windows AD server.
Winbind is the preferred method of authentication.
Read man smb.conf in detail. It is worth going through this thoroughly and understanding this if you want to run a fileserver. Also look up the howtos on Samba authentication off a Windows AD server.
For security RHEL 5/CENTOS 5/Fedora Core 6 have SE Linux. However you should always install setroubleshooter if you run SELinux in enforcing mode. For SuSE you have AppArmour which protects individual server applications.
Just as an FYI, live upgrades from Centos 4.x to 5.0 don't seem to work to great. I'll be trying a offline upgrade here soon, and I'll post back with my experences.