Random Access Memory (RAM) is very important as it is your computers most common memory that can be used by programs to perform necessary tasks while the computer is on (see Definitions of ram on the Web).
When choosing the right RAM for your computer it may be confusing as there are different types, but the RAM you purchase will primarily be determined by what your motherboard can support. When purchasing a mother board you can usually check details on what it supports when ordering, or go to the manufactures web-site and check for yourself. My recommendation when choosing a motherboard is to choose one that supports DDr3 RAM, not DDr2.
Also when choosing RAM you have to be concerned with how much memory is reasonable. I would recommend getting 2GB or more RAM (Windows 7 (64-bit) requires minimum 2GB RAM).
G.SKILL Ripjaws Series 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800) Desktop Memory desktop memory for my build. 4GB seems to be the right amount of memory for me to play games and use all of my programs flawlessly. And if I ever need more, I still have two slots left in my motherboard to do so.