You can read about their specs on the Asus web site...
Basically the A7V-266E is the "high end" model using the Via KT266A chipset. It usually comes with RAID and a nice C-Media audio chip on-board.
The A7V-266M is their cut down version that only has 2 PCI slots, no RAID, and uses a cheap AC97 audio controller.
I don't believe there is a "C" model... Asus has several other models using the "C" letter, but not based on this Via KT266A chipset.
700 Mflops in SETI!