The difference is that ATI is the actual official producer of the X800, and is usually more expensive. Other brands make duplicates of it with their own options and specifications. What to look for is pixel pipelines, memory speed, and basically bigger numbers. The X800 comes in different versions, one with 8 pixel pipelines, 12, and i think 16, the bigger the better. Also, 128 or 256mb memory, bigger is yet again better. The X700, is one version lower than the X800, basically the X800 is better. PCI-Express is twice as fast as AGP.