XPS 8900, adding internal SSD or 3.5" boot drive

No personal files, just Windows and Program Files.

I actually have a WD 1TB HDD that was brand new in an Optiplex that the power supply died in 2015. That's when I bought the XPS. I can always pull that from the Optiplex and put it in the HDD #2 bay with a SATA cable, Format the new 1TB drive i the XPS then clone it from the 256GB SSD and change Bios to boot from the new ITB HDD Drive.

I have a spare SATA Power cable in the XPS.

Do you have lots of personal files on the 256 GB SSD? You might want to move all those files onto the HDD and tell Windows (what version?) to move its default folders to the HDD and maybe free up enough space on the SSD so you won't need to install a bigger SSD. If this is Win 10, read this. 

Browsers like Firefox can also be made to move the user profile(s) and other files to the other drive. Microsoft Office has options to change the default drive where files are saved too.

Was Dell thinking you wanted to install an M.2 SSD, which might be limited to 512 GB...?

You could get a  2.5" SATA3 SSD that's up to 2T and a mounting tray to fit it into one of the 3.5" hard drive bays. There should be a spare SATA power cable from the PSU, but you will need to get a SATA data cable. Crucial, Samsung and others sell compatible SATA SSDs, so shop around...

Once you clone the existing SSD onto the new drive, you'll probably have to initialize that drive using Disk Management in Windows to remove all the OS files so you will be able to boot from the new SSD. But don't initialize the old drive until you're certain the PC boots correctly from the image on the new SSD.

What if I kept Windows on the SDD and left it as the Boot Drive and moved all the Program Files over to the new 1TB?

I would have 3 Drives:

SSD for Boot (C)

1TB for Programs & Program Files

2TB for Data

That should work, but performance will likely lag if you go from booting the PC and running apps from the SSD to using the 1T HDD to boot Windows and run apps. I guess you can always try it out, and if you don't like the performance, then consider getting a big SSD...

If this is Win 10, the Boot Manager controls which is the boot drive. After you clone the SSD onto the new HDD, reboot and start tapping F12 when you see the Dell splash screen. Select the option to boot from the new HDD.

Assuming it boots ok from that HDD via the F12 menu, physically remove the old SSD, and reboot normally. The Boot Manager should reconfigure itself to make the HDD the boot drive. Test that a few times.

And when you're sure the HDD boots ok, you can reinstall the SSD and initialize it to remove the OS (and all other files), and use it for extra storage or backup.

You can use Macrium Reflect (free) to clone the SSD and move the image onto the new HDD (or SSD).

I just ordered from Dell the Samsung Solid State Drive 860 EVO 2.5" SATA III 1TB - MZ-76E1T0B/AM drive to clone to from the 256GB SSD. I'll grab a bracket off of Amazon to install it and I'll stick the internal WD 1TB in the chassis for extra data storage for now

Sounds like you're going to keep all your apps on the new 1T SSD along with Windows. Make sure when you clone and image the new SSD, you extend the partitions so you can use all the new space. Otherwise, you'll be stuck with only 256 GB usable on the new SSD. Check the instructions for whichever software you use to do the cloning. And I'd still make Windows move its default folders off the boot SSD to whichever drive you want to store them.

FWIW, some apps may not let you install them on a drive that's not the drive with the OS on it. And if you can do that, you'll have to uninstall the apps in Windows on the C drive, and reinstall them on the other drive in Windows. You can't just copy/move app flies over to a new drive.