The case with SiS is even a bit worse from the overclocking standpoint. The barrier seems to lie at 150FSB for most examples. I have talked to one user that is running at 160, but that is the extreme rarity. Usually stability just evaporates at 151MHz. My personal SiS 645DX board (Asus P4S533) was a rock at 150MHz and went to pieces at 151. This is typical of 645, 645DX, and 648 boards. I put a Abit BD7-II in my machine with no other changes and the system proved stable at 162fsb.
I hate to be the bearer of bad new, but MSI motherboards do tend to be problematic, as does the 648 chipset. The 645's are my favorite DDR chipsets for applications that are well served by a 150FSB limit, but 648 has proven much more toubled for the majority of users.
I have used a fair quantity of MSI boards in the last 7 years, around 500. The rate of incompatiblities and failures is indeed high on these boards, far higher than the DFI and Asus products I favor. They do have a nice price and attractive feature set, but the refinement and reliability do not measure up to other large Taiwanesian makers. Recently I used a MSI 645 Ultra-C for a customer's machine, and it has failed already. I have pretty much sworn off MSI at this point.