Fortunately the format is EATX so an EATX case will work and the screw layout is standard.
You will need to remove the HEX/TORX screws that are connecting the stock HP standoffs (meant to slide into an HP case).
XW9400 Motherboard Issues
Size: 12" x 13"
Proprietary Power Connector
Proprietary CPU Power Connector
Proprietary Memory Power Connector
Fan connectors are 4 pin so you will need some kind of adapter.........
Motherboard: Gigabyte GA-3CESL: http://b2b.gigabyte.com/products/product-page.aspx?pid=2658#dl
Dell Part Numbers for the motherboard: R1232 R1232L R2232S SERVER MOTHERBOARD GENUINE 408P9 CN-0408P9
GA-3CESL Manual Download here
RAMSlots: 16 (8 controlled by each CPU)
RAMType: DDR2 ECC 5300P........