ATAxxx is an industry standard refering to the amount of data an IDE HDD is capable of transferring. ATA100 drives moves data at a rate of 100MB/s, and ATA133 drives move data at a arate of 133MB/s. the reason people say they don't notice a difference is due to the fact that IDE HDDs don't spin fast enough to transfer that much data. in fact, the most efficient drives are based on the ATA66 standard. the popular 5400 and 7200rpm IDE HDDs don't spin fast enough to transfer 100MB of data per second as the theoretical bandwidth of the ATA100 industry standard suggests. so if IDE drives cant even transfer at 100MB/s, then there's no way they can do it at 133MB/s.