If it were me, I'd add at least two more hard drives. A smallish one around 250 to 320 GB for the System drive. I'd reinstall Windows onto that.
Then I'd break out the RAID into separate drives, using one for Project and Scratch files and one for Exports.
The second new drive would be a really big one - 1TB or more. This I'd use exclusively for Media.*
* If I had a tapeless camera, such as AVCHD, then I'd definitely build in some kind of data safety, like at least a second 1TB drive for a RAID 1. Or if I had more money, add a third 1TB and a RAID controller card to make it a RAID 3.
First before you add any drives to a Dell system check out the power supply. They sometimes configure systems without and spare capacity. Also many times Dell uses special power supplies and you cannot just buy a higher capacity non-Dell PS and use it. There is a power supply calculator recommended on this forum, I just do not have a bookmark to it.
Thanks for the guidance! My issue is that the system comes pre-configured with the RAID0 on the two HD's. I'm not sure if Dell allows users to break the raid (I'd have to completely reinstall the OS) - if there's no way to do that, what's my best option as far as adding more drives (I think I can add one Sata drive internally and one through an external port) and what would be the best way to set that up?
(I'd have to completely reinstall the OS)
Yep. But I highly recommend doing that with any pre-configured system. Most computer makers put too much crap on there, and for an edit rig, you want a tightly controlled install.