CentOS 5 :: What To Consider In Setting Up Server

I've setup a server for a small office (10 people) before with samba domain and file server with slackware. no problem there. now am tasked to setup a samba domain and file server for 300 users in 5 different locations. I am currently given a 1u rack server and I requested for 2x1tb drives that I plan to with RAID.

Some of the offices will be connected via VPN using sonicwall. Would it be ideal for remote users to login to the server in the HQ? Am not so familiar with how other companies setup their servers if they have multiple remote sites. Am also open to setting up servers in each site but because of the currently limited manpower, i'd rather have it close by so I can work on the server remotely if needed.

are there any things that I should consider or are there any good references on setting up a corporate server? the problem now is that the company uses google's email app as their email provider, I have gotten a host to do that already. But since they use email to share files, its not very efficient, hence the file server. But since there are a lot of mobile employees and employees in different sites, I would want them to be able to access those files being shared access the company or departments also.

Then I have to consider how to backup the files and other apps that the company will be using. SInce we will be primarily using linux, we would need to do virtualize some windows based apps that they are using now.