I'll put it in simple terms for graphics cards. When you have a new generation of graphics cards and several older card, but they have the same Direct X type. The high end of the older model will beat the low and middle model of the next generation. ATI uses a retarded numbering system. With nVidia it makes more sense.
Also a word of advice. NEVER BUY THE NEW LOW END CARDS. The x800 GTO kicks the x1300's ass. You always want to get eitehr a medium or high end card if you do gaming other wise your wasting money. Also it might not be your graphics card that is causing problems, you might not have enough RAM, your CPU may be bottlenecking your system, virus and trojans, etc.