Why Am i Recommended to Use Memory in Pairs ?

All Motherboards Now support dual-channel memory configurations. Dual-channel technology lets the PC access each of two banks of memory on successive rises and falls of the system clock, rather than only once during the entire clock cycle. That makes "One Slot is Filled with 2GB Memory Chip" Slower than "Two Slots Filled with 1GB Memory Chips" scenario.

The performance gain, while not double, is significant. So, if you have only one stick of memory installed, buy a second one of the same size and type. If you have two sticks installed, buy an additional pair.

Dual-channel operation relies on memory being placed in specific sockets on your motherboard. Even if you've tossed the motherboard manual, determining which sockets are paired is as simple as looking at them--the pairs are typically color-coded.

Sum it up, Planning Total 1GB means Install TWO NO's of 512mb
or Planning Total 2GB means Install TWO No's of 1GB