Confused with term 800/133MHz Memory for upgrading RAM

It simply means your system will work with either 800, 1066, or 1333 ddr3 ram. If you buy 1333, the default setting may run it at 1066. And some oem systems such as dell won't give you any ram setting adjustments. I suggest you pull one stick out and match the brand and speed. Or go to crucial's website and enter your dell system model number. They guarantee their ram to work. Also be aware that if you're running any version of 32 bit windows, windows will only utilize about 3.25 gb of ram due to a limitation of 32 bit windows. You'll need to format and upgrade to 64 bit windows to utilize all 8 gb of ram.