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Yes you can do what you are thinking of doing. The Studio XPS 7100 has an ATX case, and the motherboard and components will fit into a standard ATX Mid tower case.http://support.dell.com/support/edocs/systems/Studio7100/en/sm/index.htm
Dell is known for having non-standard mounting systems, and you will have to ensure that the motherboard stand-offs match the pattern in your new case. If not, you may have to drill and tap a few holes; not too difficult to do.
If you add fans, 'daisy-chain' connect them directly to the PSU using Molex connectors.
If you need specific guidance, please ask.