The AGP connector installed on many Pentium® 4 processor-based desktop boards is keyed for 1.5V AGP cards only.
The AGP connector is not mechanically compatible with 3.3V AGP cards. To some the 1.5V AGP connector might look backwards because of the connector key being exactly the opposite of the 3.3V AGP connector.
AGP 2x/4x Universal Graphics card with AGP 1.5V slot
AGP 2x Graphics card with AGP 1.5V slot
AGP is an effective interface for graphics-intensive applications, such as 3D applications. Based on the AGP Local Bus Specification, Rev. 2.1, AGP is independent of the PCI Bus and intended for exclusive use with graphical display devices.
This applies to: