ATX, Actually todays most common motherboard form factor used is MicroATX
ATX is the most common.
The shape and layout of a motherboard (as well as the case) is called the form factor of the motherboard/case. Different form factors include AT (old & out of date), ATX (most common), BTX(up and coming), as well as uncommon form factors like NLX.
Some motherboard form factors (from smallest to largest in physical size) are: -Mini ITX -Micro ATX -ATX -E ATX
The closest thing that standardizes size and shape is what's known as a "Form Factor". Common form factors are AT, ATX, BTX and ITX. The most common is ATX. These form factors specify motherboard sizes and general component placement. For example, you usually need an ATX compatible computer case if you wish to fit it with an ATX motherboard.
NLX
atx,at,btx
Three form factors are AT, ATX and BTX. AT is the older Pentium, real big with just a few connectors. ATX is the most common today, mid-size, more standard connectors for future devices, the power supply is placed differently to work better and the placement of RAM is closer to the CPU which enhances performance. BTX, is designed for powerful computers so the cooling system is improved, the CPU is moved to the front of the motherboard to get cool air and has a special heat sink and an assembly on the CPU that blows air away from the CPU.
The most popular form factors are: -Mini ITX -Micro ATX -ATX -E ATX
NLX BTX ATX
NLX LPX Mini LPX
NLX has slots parallel to the motherboard as ATX does not
The largest Form Factor was the EATX motherboard. The board measured 12 inches by 13 inches. Today the largest is the BAX motherboard which measures 10.5 inches by 12.8 inches.
Unfortuntely, the Dell motherboard form factors are propietary, so you'll have to buy replacement motherboards directly from dell.