If that Dell case requires a micro ATX motherboard (dimensions of the actual motherboard) vs. a standard ATX motherboard, it will limit the number of boards you can choose.
If it is Micro, I might just go with a new case to give you all the possibilities for motherboards and the fact that Dell cases aren't anything super to start with.
Agree with the above poster, you have to have a list of everything you need, everything needs to be compatible AND fit in the case. And that's not even considering the quality or performance of the components - it's just a "will it fit in the case and work together" analysis.
Building a PC is like cooking from a recipe. You have to have all the ingredients, follow the directions, execute correctly and even then you might not LOVE the taste, even if done correctly.
Make sure you have all these ingredients covered and choose the motherboard and processor after careful research.
Motherboard
Processor
CPU Fan
Ram
Video Card
Hard Drive
Case
Power Supply
Floppy
CD/DVD drive
Network Card
RAID Controller
Monitor
Sound Card
Speakers
Operating System