Have you updated the Motherboard bios to the latest revision? That may be needed before the new GPU will function correctly.
The best place to ask your question is with Dell, contact them to discuss your options for upgrading your GPU.
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OEM systems sometimes have a more restricted BIOS then retail motherboard vendors, therefor I would first try to update your bios version. If this doesn't help you should contact dell to ask which GPU's are whitelisted for your pc.
Try the r7 250