BTX is not compatible with ATX cases. Even if the screw holes line up properly the placement of the i/o panel in the back of the system is different and will not match up to an ATX case. Basically the i/o panel is on the bottom with BTX as opposed to being on the top in ATX.
Another thing is the official BTX standard was dropped from further development in 2006, so any system built with a BTX-like form factor is a mutation from the BTX standard to a proprietary form factor after official development was dropped.
Even if your board is not a true BTX board, which it looks to me like it is not, it is still a Dell proprietary board and will likely not fit in any case except the one it came in.
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