Command Rate is the amount of command (cycles) it takes for a RAM module to react to a command from a program or user. 2T and 1T are how most modern modules are classified. 1T is the faster rate. Think of it this way. When you click to do something in Guild Wars, that command is issued to the processor and depending on availability, it is issued to the RAM at some point. That information reaches the RAM, but it takes either 1 cycle (hertz) or 2 cycles (hertz) to trigger a reaction. Now, considering DDR2-800 runs @400MHz x 2 as explained below, that means that it would take approx a 8th of a millionth of a second for the RAM to respond if the command rate was 1T, and a 4th of a millionth of a second if it was 2T. Sounds like a very small number, but add it up... If Guild Wars issues 600 million commands per second to the processor, the RAM has to do that. That can mean the difference between 20 frames per second (FPS) and 40 FPS. Now it sounds more important right? If you are comparing two modules of RAM, one having a 1T and the other having a 2T command rate, and your budget can be pushed or allows for the 1T, go with that. Motherboard settings can be tweaked to allow for faster performance in 40% of the applications you will use because of command rate.
Ref : http://guildwars.incgamers.com/forum.../t-442997.html