Good brand memory is usually tested beyond its specifications, eg at higher speeds than 133Mhz for PC-133, to ensure that it can run at 133Mhz "in it's stride". This usually means that the memory will be able to run higher than 133Mhz with ease. Lower latencies will also often be the case, such as CAS 2 rather than CAS 3. Also, some companies "hand pick" memory modules for the very best performance, thus you get things like PC-150 or PC-166. Another factor is not the memory chips themselves but the PCB they are mounted on. Generic memory will very likely be of lower quality in this respect.