Dell servers Mac friendly?

The "Dell" part has nothing to do with it really. It's the OS you'll be running on it that plays a role.

Are you installing or buying Redhat Linux or Suse Linux? This is completely different from say Windows.

Are you having to build a NAS box for user shares, or are you needing a domain controller (Active Directory)?

I think both are possible with Windows when using Mac clients, but some admins may find a Linux server easier to work with when using Mac clients (or a mix of Windows and Mac clients).

I guess if I want to run Mac and Windows, I'll have to install Linux.

Not sure what a NAS box is.

NAS stands for Network Attached Storage.

This allows users to put their files on network shares (on a NAS system).

A NAS is an implementation that can be done with a regular server as well, however a NAS system is dedicated (usually) to just the task of making network disk space available to users, where a server can (usually) do more than just that (can also be a domain controller, firewall, database server, email server, etc).

What were you wanting to use the server for? Are you wanting to set up a domain where you can regulate security and such, or are you more looking for a system that can hold user files and such that can be accessed by multiple users/computers?

Good questions. It's both actually. I am looking for security in the system obviously, but we deal with very large graphic files -- sometimes as large as 100 MB. We also need to transfer those files pretty frequently, so it would need to be set up for FTP as well. Would that be possible on this type of server? This is obviously out of my field of expertise and I'm operating totally in the blind here, so I appreciate your advice.

Any server (Dell Poweredge servers included) can meet your needs. However, the OS you run on it can influence compatibility with the clients you have.

I'd suggest contacting Dell sales and talking to one of their technical sales reps about your needs and see if Redhat Linux, Suse Linux, or Windows (2003) Server is the better choice. I'd also recommend paying extra to have Dell come in and set it up for you as you want to make sure it's done right the first time so you don't have to spend a lot of time on the phone with Dell getting your system working (warranty covers part failures, but doesn't cover sending a tech to set the system up, unless you paid specifically for an install).