Best OS for Cpanel with HP ProLiant DL145 G2 SATA Server 2.6Ghz Opteron(Dual) 4Gb 80G

Title says most of it.

Have a new HP ProLiant DL145 G2 SATA Server 2.6Ghz Opteron(Dual 252's) 4Gb 80G

I'm looking for the pros and cons of using the x64 OS's that Cpanel supports.

Building a hosting server.
I would like the following features.

LDAP to support aliases when using a Barracuda to Scan/Filter incoming & outgoing mail.
I hate Front Page Extensions but some sites need them.
SMTP authentacation/relay.

And all the standard perl, gcc etc. etc.

MySQL DB's

And finally ease of use and daily maintance.....

And does anyone have any guides as what I packages I need to install with the base OS and those I can safely omit and save the disk space if it will never get used.

Greg