Dell tends to customize the motherboards and cases that are used in their computers, making it hard to replace the motherboard with anything than a replacement of the same model. If it is an Inspiron you are trying to upgrade, that's a laptop, which is even worse when it comes to upgrading, since very few parts are standardized.
You still have the problem of the case not taking standard ATX motherboards. However, you may be able to use parts from an XPS 410, since they were related to each other. Some XPS 410s were even sold as Dimension 9200s.