Hardware considerations for mail/directory

[Maykel Moya <moya@infomed.sld.cu>]

I have to plan a deployment of a descentralized mail and directory
system. The system should handle at least 250k and should scale to
about 500k accounts.

The mailboxes will be in a storage (NAS) attached to many front servers
(IMAP & POP). I have to choose between:

1. HP DL360 G4 2 GB RAM 2 x Xeon or
2. HP DL560 G4 6 GB RAM 4 x Xeon

The cost of DL560 is about five times the cost of the DL360 (last time
I checked). I thought the best tradeoff is to maximize the server count
so I think I'll opt for DL360s. Any comments?

What do you recommend to do load balancing in terms of benefits/cost.
Is it better to have some equipment (hardware) to do the balancing, or
a software solution (HA + director) is pretty enough?

For directory, I'm thinking in 4 servers, 2 DL360s HA clustered to host
the master LDAP and two DL560s to host two LDAP replicas. I choose
DL560s here for the amount of RAM available. It is better here to have
many DL360s instead of two DL560 for replicas?

Examples of services you're managing, which servers (CPUs, RAM) to give
which services and so on are more than welcome.

Regards,
maykel