New CPU/Mb in Dell XPs 720 system

Hello Valar;
Your XPS 720 uses a BTX motherboard and case. You won't be able to slip an ATX P55 motherboard inside the BTX case.
Did you get the 750W power supply in your 720? How old is it?

Your operating system will have issues with the new board, so you'll have to install fresh... and this may or may not be allowed depending on the version of the OS and Dell's license.

Yes it's very likely the Vista will be OEM and so a MB change will require that MS signs off on the re-activation. If you explain to them that the old board died, they will probably be OK with it ... unless Dell wants to sell you a replacement real bad.