Dell Dimension 8400 is LGA 775. The 8400 was only obtainable as such. Easy as that. To be honest, I'm not certain why I require telling you this. If you certainly did effort to insert a dual core Celeron into your Dimension 8400, then you would already identify actual that it is an LGA 775 socket. The Celeron fit within the socket, didn't it?

The 8400 holds only Celeron D (apart from 356, 347, 360, and 352 and all wrongly labeled 400 series Celerons) and Pentium 4 Prescott CPUs of the 5xx and 6x0 diversities. No dual core CPUs will work, not even Pentium D. This is doing to a grouping of BIOS restrictions, outdated VRM revisions and chipset confines.