This guy is RIPPING YOU OFF!! He is trying to make a sale, pad his profit, and clear his older inventory of PC100 RAM while he can. Do "NOT" buy from a local retail store, ever! Buy from the internet. I rarely have any problems with any of the parts I order, probably $20,000.00 worth in the past 4 years from www.MWave.com. The only thing I believe this guy is trying to get is your money, not a happy customer, or this guy is just plain ignorant and you know more than he does! Either way, buy from the internet, you won't be disappointed. PC133 RAM is not so much as needing AMD approval, but the motherboard needs to support PC133 memory, and almost all motherboards that support AMD Duron's and TBird's support PC133 RAM, you just won't be able to run the CPU at 133mhz FSB, front side bus, unless you get a motherboard equipped with the VIA KT133A, "A", chipset that supports 133mhz, even though AMD does not make an official Duron CPU that supports 133mhz FSB, it will, as I have my Duron 700 running at 933mhz, 7x133mhz. If you don't know what this means, keep reading all of the posts and websites and you will figure it out. I believe www.Anandtech.com actually reviewed a Duron at 100mhz and 133mhz to test performance gains, and he did not report any problems at 133mhz. I hate resellers that blow smoke up people's asses, they only make an ass of themselves when you learn you get screwed. My advice, kindly tell this guy thanks for his incorrect information, and leave his store and don't go back, ever! This is not the way to treat customers. If he knew what you were telling him was correct, he should admit that and sell you the better parts. The only thing here is the power supply, as AMD recommends approved power supplys only, especially for higher clocked CPU as they pull more from the power supply, and that is what you really pay for with most cases. Most generic cases have very cheap power supplies in them, as I have seen many POP, Sizzle, and Crack. If you look at AMD's site they have all of their approved power supplies listed. It would be funny for you to take a printed list of these to the store with you, and have that guy show you his approved case, and investigate the power supply. You don't have to have an approved power supply, but it will guarantee you that it will be able to supply power and not blow up..