Motherboards have a size rating. Most are ATX or Micro ATX. An ATX case can fit both, but a MicroATX is too small to fit a standard ATX motherboard. The other thing you need to know is if the power supply will work. Most P4 boards have an extra power connector to supply extra power to the CPU. You need to make sure the power supply has this connector. Also the wattage of the power supply is pretty imortant. Chances are if you have an OEM computer with an i810 motherboard, then it is a budget box with the smallest power supply they could get away with. Likely it is 200w or less. This will not do. You normally want at least 300w for todays CPUs and some might even need 400w to be safe, especially if it is a genaric power supply.
A good case and power supply is Antec. Their Performance Series is very good, but a bit pricy ($80-120 or more)
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