your performance won't necessarily go down. it's just a matter of whether or not your pc66 ram will work at fsb100. if it does, that means that you could theoretically consider them pc100. it's just overclocking...for example, overclocking a celeron300a to celeron550 will not, of course, make your performance go down. it goes up. again, it's just a question of whether not your ram can handle it.