Most desktop chassis come in one of two form factors: ATX or microATX. Systems based on the Pentium® 4 processor, in either the 423 or 478 pin packages, must use chassis that comply with the ATX specification (revision 2.01 or later) or microATX specification (revision 1.0 or later). The specifications can be found at www.formfactors.org.
For a Pentium 4 processor-based system in the 423 pin package, the chassis must also have Pentium 4 processor-specific mounting support. This includes four additional holes located in the base plate of the chassis, four removable standoffs, and four screws. The screws must be long enough to securely engage the standoffs when installed in the retention mechanisms through the system board. For a Pentium 4 processor-based system in the 478 pin package, there are a wide variety of solutions available today which provide motherboard and processor mounting support but are independent of the chassis and do not require additional holes to be drilled.
Allow room for spare drive bays and space to work inside.
If you plan to do a lot of upgrading, look for a case with easily removed motherboard mounting plates and drive racks. Depending on where you want to keep your PC, you will need to decide if you want a desktop or tower case. Also, be sure the case is sturdy and comes apart easily -- though not in too many pieces.
The chassis for a Pentium 4 processor-based system must also support a lower internal ambient temperature than many standard ATX and microATX desktop chassis. The internal temperature should be maintained at less than ten degrees Celsius higher than the air temperature outside of the chassis. Look for chassis with rear exhaust fans to improve airflow. 80mm chassis fans with 3 pin main board headers are recommended.
Chassis for Pentium 4 processor-based systems that include an installed power supply must support the ATX12V or SFX12V design guidelines.
Your chassis solution must have support for the processor heatsink assembly and additional support mounts for the heatsink. Therefore, for Pentium 4 processor-based systems, it is recommended that you use a chassis that has been designed to mechanically support the Pentium 4 processor.