I don't think it's necessary to take more than 100% RDA of C with MSM. I think it's the ratio between the two that matters. According to the En Garde website a ratio of 1:10 Vitamin C to MSM is what experts consider optimum, eg 100 mg C to 1000 mg MSM. But I also read on a few other websites that although Vitamin C works best with MSM, it's not essential as we've been led to believe for MSM to work. This is one such website.
BTW, for those who have a problem with acid stomach as I do, Ester-C is the brand of C you should be taking. Not only is it pH-balanced but it is easily absorbed and time-released so it doesn't all get p-d away in one go.