Kolab with 1500 user's

[rajnish kumar]

Dear experts and Gunnar sir
                   Actually now a days i was try to testing open
source collaboration suits lastly i was stop with zimbra and kolab but
i see that zimbra is not completly open source suite ultimetly i am on
the way of kolab so dear i required some advice by all of you.

I have intel based 64 bit server. I want 1500 user's in kolab

1.  which Operating System should i choose ( Fedora, CentOs, Debian,
Gentoo ) for kolab.

2. What Partition should be used for better performance..

3. For 1500 hundred user's how much Ram required and speed of
processor required.

4. We required about 2 TB hard disk space for mails so should i go for
Lvm or  RAID

5. How kolab support  cluster and high availability .

with regds
rajnish