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More than likely it's a mATX MOBO, but to check you can measure it as follows:
MOBO Type = length X width
Standard ATX = 12 in (305 mm)X 9.6 in (244 mm)
microATX = 9.6 in (244 mm) X 9.6 in (244 mm)
Mini ATX = 11.2 in (284 mm) X 8.2 in (208 mm)
FlexATX = 9 in (229 mm) X 7.5 in (191 mm)
EATX (extended ATX) = 12 in (305 mm) X 13 in (330 mm)
EEATX (enhanced extended ATX) = 13.68 in (347 mm) X 13 in (330 mm)
WTX (workstation ATX) = 14 in (356 mm) X 16.75 in (425 mm
My general experience is Dell is very difficult to upgrade as you've described. In most instances the I/O Shield is welded to the case, and the overall case layout is not conducive to standardized MOBO, and PSU.