Partially ripped from Sunny129:
CUSL2 (i815E)
onboard video: yes
onboard audio: you must ask for it or you probably wont get one w/ onboard sound
AGP Pro slot: yes
ata100: yes
CUSL2-C (i815EP)
celeron support: no
onboard video: no
onboard audio: you must ask for it or you probably wont get one w/ onboard sound, but you can get it since its not part of the i752
AGP Pro slot: maybe...i heard from someone in the forums that they advertise the CUSL2-C w/ an AGP Pro slot, but when he got his mobo, it only had the standard AGP slot
ata100: maybe...this is also something that they advertise with the CUSL2-C since ata100 is part supported by the i815EP chipset, but i've heard of people who got their C's and with only ata66 support...go figure
CNR: 1 less than the CUSL-2