Funny enough, I have one of these particular boat anchors sitting on the floor right here, right now. (I need to get it packed up and posted on eBay. Anybody want a PowerEdge 2600? I'll autograph it... heh heh...)
I put some thought into making it a "useful" machine, but realisticly there's no internal storage option that works well since you'd have to butcher the hot-swap backplane and, very probably, eliminate any hot-swap capability in so doing.
The box is extremely power inefficient compared to a modern box in terms of compute operations per watt. You can make a great business case for chucking it and buying a new machine that is much more energy efficient and will use SATA hard disk drives.
Edit:
If you want to put SATA drives into the sleds that come with the box you're going to have to remove part or all of the factory SCSI backplane. In the world that I work in a server computer that's had factory parts forcibly removed from it to add new "features" isn't a reliable server computer anymore.
You could get a controller with something like eSATA ports. If that controller had BIOS and RAID functionality you could certainly use it. You might have to disable the option ROM on one or the other, but you could boot off of either (assuming you can find an eSATA controller w/ RAID functionality and a bootable option ROM). (AFAIK that server doesn't have PCI Express slots. My box has things stacked on top of it right now and I cannot be bothered to look. That box is vintage 2003 - 2004, though, and I'd be shocked if it had PCI Express slots...)
Get a PCI Firewire card and stick some external disks on it if you absolutely have to keep using the machine.
SATA or SAS are the way to go.
IDE and SCSI models are usually older, smaller, slower drives. SCSI is also vastly more expensive.
Get a good (not cheap) RAID card and do a Raid 1 at least. In case this is too expensive, I would prefer OS-based software RAID over cheap raidcard or onboard RAID.
Also make sure to buy enterprise or nearline disks. Standard disks are not meant to run 24/7.
Best Answer
Typically RAID1 isn't anything more than two normal drives with a bit of metadata at the end of the drive that the hardware RAID controller uses.
So it should be safe to put one of them in another computer. I wouldn't use a RAID controller for this just to avoid any potential for it reading and changing the RAID metadata in any way. Plus you typically have to do some low level RAID setup stuff for it to see a new drive which I would try to avoid if possible. But chances are that on a regular SCSI controller the drive will work fine and you can copy your data off.
If you want to be on the really cautious side just use
dd
to make an image of one of the drives and then mount the image using a loop device on Linux.