Well, here's the owner's manual.
No mention of form factor in there. They refer to it a "system board".
Be very wary of upgrading pre-built, big-name boxes like this. DELL ESPECIALLY. Gosh I hate dell. They're often proprietary form factors and even simple hard drive or RAM upgrades won't be compatible (unless you buy their proprietary components directly from them). Much less a graphics card. Dell is notorious for this crap.
Well, well, well lookie what I found - Dell proprietary b/s.
Here's the link to their own support community with a verified answer. Did I mention Dell sucks?