Nice job on the responses!
Until you do your own encoding, you can get by with a lower speed CPU.
The Athlon XP\Nforce combo will be good choice for a fileserver.
If you really plan on doing "Content Creation" (I love that term) then a speedy P4 would be better, unless you're going to us an ATI All-In-Wonder card or a Matrox RT-series card.
An Intel 875 chip has RAID capability, so you could run a couple small drives from that, and use the multi-port RAID card for the data array(s).
I can see a few options on the storage (there are dozens of combinations)
1) A 4-port ATA RAID controller w/ 4 160GB drives in a RAID 5 array. Low upfront cost.
This should give you a ~480GB volume that you can use for the OS (4-8GB) and data.
In the future, you could add another controller and up to 4 more drives to expand things.
These cannot be combined into an 8-drive array, however.
2) As already suggested, an 8-port S/ATA RAID controller w/ 2 drives for OS (mirrored) and 4-6 disks in a RAID 5 array. You would need 6 drives to start with (2 small, 4 large), and room for two more.
3) A 8-port S/ATA RAID controller. Start with 4 drives in a RAID 5 array. Then you can add 4 more to the *same array*
-->If plan to use applications on system, with a modest amount of RAM( e.g. 512MB), then RAID5 for the OS may not be the way to go. Depends on how much you use the apps.
You *could* run the OS of a standard drive attached to the motherboard IDE ports--and use RAID if available. But carving a 8GB chunk out of a 500GB array seems like the better way to go.
Using the onboard controller does give you flexibility to swap arrays and boot whether the RAID card is functional or not. *shrug*
OS: I have used Samba a lot in the past. I don't know how Samba integrates into the AD Tree (other than as a NT4-style member server). The Samba group (and others) are claiming similar, and in some cases better, windows-share performance as compared to W2k3Server. So, unless you already own W2k3S, or want full AD functionality (I don't see the need for file services), give Linux (RH8/9, etc.) a try.
Others here will have experience with the drive, controller, OS combo you're interested in.
Take a look at equipment prices, look at the performance database to get the best value on drives (160GB seems to be a nice price point...200GB is close), and post you're thoughts.
We're all full of experience and opinions.