If you plan to do hard work on the CPUs, then yes quad would be best.
One drive is fine for OS. If you need storage and it's important data, go with 2 data drives (backup + backup of backup) or a RAID array. Cards can die and you'll never recover that data. Backup the array external to the card however you like. It just depends on how much you like your data. I like mine alot, I have data going back to 1999, so I back up and back up the backups.
Xming runs in windows as a library resource for X. X is the backend "server" for the frontend window program in linux and unix, such as fluxbox or KDE. puTTY is the SSH tunnel that sends and recieves the X session data on the windows client. You can open a puTTY session to the machine, run Xming as the library resource, and type "firefox" (provided you installed it) and a firefox window will pop up on the windows machine, with the settings and data being taken from the freeBSD box.
Bear in mind freeBSD is not easy to install. If you are used to it you can do it with your eyes closed, but it takes alot of time and research for someone new to figure it out. You NEED to familiarize yourself with the unix/linux command line first. You don't have to be a programmer, but you do need to have a basic understanding of certain commands. The man pages help alot, even for veterans.
Also bear in mind if you need to run a specific application, it more than likely will not be ported to freeBSD. It might be if it runs within linux natively, but make sure of these things before you get the OS up and going. Also, samba sharing will be a huge pain to configure if you've never done it before. Even I hate messing with samba configs.... Windows programs DO NOT run in unix or linux without emulation (it's a crapshoot, use "wine") or unless they are ported, like firefox.
FreeBSD uses packages. Familiarize yourself before you even try the OS. It's imperative that you know how to use the ports collection as well (and go ahead and install it in the beginning).
If this sounds daunting or you aren't willing to research for, say, the next month (you will be google's best friend by the time it's running to your liking
), then 2003 will make your life much, much easier. It's limited to 3 sessions, but it's already set up from install and it's already familiar.