CUSL2: i815E chipset with onboard i752 video, 7USB ports, 2CNR, AGP Pro slot.
CUSL2-C: i815EP chipset, no onboard i752 video, 4USB ports, 1 CNR, Standard AGP4X slot, no support for PPGA Celerons.
AGP Pro slots provide either 50watts or 150watts of power to very high end graphics cards....some nVidia Quadro cards use them. There are currently no consumer level cards requiring AGP Pro slots. Difference between these and AGP4X slots? No speed difference. AGP Pro is simply a 4X slot with 50 extra pins for additional power.