ATX, introduced in late 1995, defined three types of power connectors:
1) 4-pin "Molex connector" transferred directly from AT standard
2) 4-pin Berg floppy connector — transferred directly from AT standard
3) 20-pin Molex Mini-fit Jr. main motherboard connector — new to the ATX standard.
A supplemental 6-pin AUX connector providing additional 3.3 V and 5 V supplies to the motherboard, if needed. This was used to power the CPU in motherboards with CPU voltage regulator modules which required 3.3 volt and/or 5 volt rails and could not get enough power through the regular 20-pin header.
ATX12V
Use of a 24-pin Main Power Connector over 20-pin Connector for PCI Express Support
6-Pin Aux Power Connector Not Required
Use of Dual 12V Rails if Greater than 20A
Serial ATA Power Connectors Required
ATX12V 1.x
While designing the Pentium 4 platform in 1999/2000, the standard 20-pin ATX power connector was found insufficient to meet increasing power-line requirements; the standard was significantly revised into ATX12V 1.0
ATX12V 1.0
The main changes and additions in ATX12V 1.0 (released in February 2000) were:
Increased the power on the 12 V rail (power on 5 V and 3.3 V rails remained mostly the same.
An extra 4-pin 12-volt connector to power the CPU. Referred to as the P4 connector because this was first needed to support the Pentium 4 processor.
ATX12V 2.0 (introduced in February 2003), up to the current version of the ATX12V spec, published in April 2013 - ATX12V v2.4
Since you provided no information concerning your hardware, I wouldn't know what you should get.