If you are upgrading the case, motherboard, and ram the rest of your stuff should work like a champ. I do have two suggestions for you.
First, Dell uses an unusual CPU cooling scheme. The heat sink does not have a fan on it because there is a cooling duct that runs from the heat sink to a fan mounted on the back of the case. It's improbable that you will be able to rig the same setup in your new case. Your CPU will be depending on just the heat sink to dissipate heat. I'd purchase a new heat sink with a fan mounted on it.
Second, if you replace the motherboard then I would recommend a fresh install of your OS.
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