What is the "physical difference" between PC100 and PC133? Another word can I use PC133 instead of PC133? I am not worried about the speed being compatible. Rohn

Pc100 can operate functionally at no more than 100 mhz and pc133 can operate at 100 mhz or 133 mhz. If your fsb is running at 100 mhz and you use pc133, the pc133 will operate at 100 mhz. If your fsb is operating at 133 mhz and you are using pc133, the pc133 will operate at 133 mhz.

PC100 is designed to run at 100MHz, but it actually "should" run at speeds up to 125MHz PC133 is designed to run at 133MHz, but "should" run at speeds up to 142MHz. http://www.dewassoc.com/performance/memory/pc100_std.htm There is no real "physical difference" between PC100 & PC133. Both have 168-pins & can be either single sided or double sided. The big difference is compatibility. PC133 can be run at 66MHz, 100MHz, or 133MHz (or anything in between), but newer PC133 is built using "high density" memory chips. Older motherboards will not recognize high denisty RAM properly...either it will be recognized at 1/2 it's capacity (ex: 128MB seen as 64MB), or it won't be recognized at all. The best thing to do would be to go to a memory site that has a "memory configurator" (such as Crucial.com) & enter your motherboard make/model & see what they recommend. If you use PNY's memory configurator, you will be greeted with this statement: "PC133 Modules are NO LONGER backward compatible with PC100 Machines. To determine the correct memory upgrade for your machine, use the PNY memory configurator." http://www.pny.com/configurator/ http://www.crucial.com/index.asp ASUS A7N8X-X Athlon XP 1800+ 8.5 x 200MHz 1024MB PC3200 2.5-3-3-7 Asus A9550GE/TD 128MB WinME/WinXP Pro SP1