I haven't seen any review on the net as yet. Only VIA had released their datasheet and some information comparing the KM133A with the KM266. The KM266 is essentially the KT266a + ProSavage8 (doh, that's what VIA calls it). The graphics core nearly same as the older prosavage except that it can transfer @AGP8X (internally). The beauty of the KM266 is that using SMA on such a board won't hurt performance greatly, due to the KT266A's good performance. I'd "guess" a 5-7% hit on memory, when running 1024*768*16. That would put a KM266 board slightly ahead of an AMD760 board in memory performance, even when using SMA. I doubt there's any change in graphics core, except that the Prosavage benefits from the additional DDR bandwidth. VIA says 3D games performance improve by about 30% vis-a vis the KM133A. Overall it's an excellent value solution.
Currently I know only about the Gigabyte 7VKML board that uses it. I've heard at AMDMB forums that the board is excellent and offers decent performance...Integrated graphics, audio, LAN, ATA133 etc.