We are going to do a multi-server test environment and wanted to know from community experience what servers should be the most "powerful" and which server can be on the lighter end.

For this we want to setup the following:

1x LDAP
1x Mailstore
1x MTA
1x proxy
1x archiving

Once this is proven to work we are going to want to add additional LDAP, Mailstore and MTA servers. Once tested to work this will be deployed rather quickly and on a large scale to host email for our clients. So it must have scalability in mind.

We have the following machines available for test and will purchase similar server grade systems for deployment.

1x 8 core (upto 16gb of memory)
2x 4 core (upto 8gb of memory)
2x 2 core (upto 8gb of memory)

Also, from community experience is Linux soft-raid an acceptable solution?