Dell stuff isn't designed for upgrading. Same for other oem brands. You're practically building your own already, and your dell copy of windows won't work with a non dell board. If your new ram doesn't work with the dell board, you might want to get a non dell board, and another copy of windows. Or simply return the ram and pay a restocking fee. Crucial.com guarantees it's ram to work if you use their configurator and order from their website. You pay a little more but I don't think you have to pay any restocking fees. I buy at frys; they never charge a fee to return ram within 15 days.
What I don't get is that other people here have upgraded their xps 420's. I said in the original post that I put the stock ram back in, so it wasn't the ram obviously. And it's not the OS because I have Windows 7 Ultimate which I purchased. Plus it wouldn't matter about the OS anyways because it doesn't even get to any screen. I can't even enter the BIOS. I checked crucial's website before buying to see if the ram was compatible and it was.