A Dell XPS? Unless they've recently changed their policies away from proprietary form factors, you're fucked. There's nothing that will fit the case properly except a Dell-manufactured motherboard, and they don't sell those (unless you send it in for THEM to repair).
I'm not 100% certain about whether or not Dell still does this, but regardless, the next time you purchase the computer,
build it yourself. It's cheaper, you know EXACTLY what goes into it, and you KNOW that if anything breaks you can fix it.
Sorry about the sudden rant. It's one of my pet peeves.
Jam it back in, in the dark.