The easiest way is to download a simple software program that scans your system hardware. We recommend CPU-Z. Find out more here or you can find the download here
Once the scan is complete, there should be noted in slot what memory is installed. One of the numbers should correspond to the descriptions above. Not sure? Call our staff on 1800-289-726.

RAMBUS RDRAM was originally available at four different speeds, ranging from PC600 upto PC1066. Today, the slowest speeds (PC600 and PC700) are quite difficult to source and as a result, Upgradeable only offers the PC800 and PC1066 RDRAM. If you computer is running either of these two slower speeds of RDRAM, you will be able to buy the PC800 speed memory, as it will clock down to match your existing RDRAM or Memory Bus speed.

This is a common misconception. A faster module will not run faster. It will run as fast as the slowest memory installed. So if you have 800Mhz installed, a 1066Mhz memory will only run the speed of a 800Mhz memory RAM. However, extra memory will make the computer run faster!