No, the PC133 is just rated to work at a maximum os 133Mhz or 7.5ns. However yoyu may get problems mixing old ram with new if you use diffrent speed ram (e.g Cas 2 + Cas 3) but i have mixed PC66 and PC100 (all cas 3) without any problems.
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NOt normally a problem. If you have the Intel VX chipset it won't work. If you have an old motherboard it may be limmited to 64 or128MB modules. So unless it's an Intel VX, you shouldn't have a problem using 64MB PC133, or 128MB PC-133 in most situations.