Graphics cards come with various types of memory technology, or RAM. The type of RAM that is embedded on the card will have a significant effect on the card's performance. Briefly, there are descriptions of some types, below.

DDR
Double-Data Rate RAM essentially doubles the rate of standard SDRAM. Any DDR RAM will be quicker (and probably more expensive) than its non-DDR counterpart.

RDRAM
RDRAM basically doubles the output of SDRAM and also moves the data quicker. This is a high-end variant of RAM.

SDRAM
This is a high performance, faster variant on some other types of RAM. The S stands for "synchronous" because it uses a new technology to send spurts of data at specific intervals of time.

SGRAM
This is a graphics-specific type of memory used in high-end graphics cards. This type of memory sends data in blocks so that graphics appear less choppy when read.