(Asus cusl2) is the standard 815e chipset that has built-in video like all 815e chipsets. The built-in audio is optional for this motherboard, meaning the store can choose to buy & sell these without audio, most places sell without audio because the onboard audio is so cheap, hardly anyone wants to pay extra for it.
(Asus cusl2-c) cost less than the basic Asus cusl2 because it doesn't support ppga, only fcpga format cpu & it also has a optional audio which most suppliers sell without audio to reduce cost, besides that it's pretty much the same as the cusl2 but cost $20 less.
(Asus cusl2-m) is a micro-atx version of the cusl2, designed for mini computer cases, & has built in audio, but has a optional built-in 3com lan which some suppliers might choose to sell without.
All 3 models can be upgraded, that's what the agp slot is for, when you plan to upgrade your videocard, just disable the onboard videocard 1st & place in your new videocard, same applys to the built-in audio, but you don't have to worry about that, since hardly any place sells it with onboard audio.
I decided to buy a 815e motherboard for someone for christmas, because they have a pentium 200. I picked the microstar 6337 815e pro, even though I have never brought this brand before, I read some good reviews on it, & it was 2nd best compared to asus cusl2 on the anandtech roundup, it's easier to find a microstar 815e with built-in audio/video & the cost is much lower. This is the 1st time I brought a motherboard with built-in video/audio only because it's for someone else, with this option I save money & they save money too, because they don't have to worry about a new videocard or audio right away.