RAID & LUN design for Multi-Role server

Hello All,

I'm having a bit of difficulty trying to come up with a suitable Mailbox design for our new Exchange 2010 server.

The server will be Multi Role with near on 900+ users moving across from Exchange 2007 with now HA or DAG.

The current available disks are 4 x 7.2K RPM SAS 3.5"

The three options I was looking at was as follows:

4 x Disks at RAID 5 then one LUN for 4 x Databases and one LUN for Log files

or having

2 x Disks RAID 1  for one LUN for 4x  Databases

2 x Disks RAID 1  for one LUN for Log files

Or finally 

Having tried to use the Exchange Mailbox Calculator;

Buying 2 more disks then having the current 4 as RAID1 for 4x databases each on there own LUN then 2 new disks for Log files.

The following are my user criteria:

157 x with 1.5GB mailboxes 

105 x with 750Mb mailboxes

717 x with 100Mb mailboxes

All averaging 50 x 75k messages per day

Many Thanks

http://blogs.technet.com/b/exchange/archive/2011/04/08/robert-s-rules-of-exchange-multi-role-servers.aspx

http://blogs.technet.com/b/exchange/archive/2007/01/15/3397742.aspx