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800mhz fsb cpu not same as 400mhz mainboard socket
(most likely LGA775 CPU vs Socket478 MB)
Socket 478 will suppoprt up to P4 CPU's with 400mhz FSB.
CPU's with 533+ MHZ will not fit Socket 478 (this covers all dual-core CPU's)
However, if, when using a CPU with a higher FSB value (IE, e8xxx-series CPU w/1333 FSB on a 1066 MHZ FSB mainboard), the system should work without damage to the CPU, though at reduced performance (therefor is not recommended.)