The two speeds listed are Memory speed and VPU/GPU speed. Just like a faster CPU runs programs faster a faster VPU/GPU will be able to draw more pixels per second. If the VPU/GPU is too fast then it will spend all its time waiting for textures to be read from memory, which is why you want fast memory too. In addition to memory speed the memory can be hooked up to a 64bit, 128bit, or 256bit bus. A 100Mhz 256bit bus would have the same bandwidth as a 400Mhz 64bit bus. Wider is better
AF is anisotropic filtering. It makes textures look better on angles. Check out these links.
http://developer.nvidia.com/object/Anisotropic_Filtering_OpenGL.html
http://developer.nvidia.com/object/Filter_Anisotropic.html
4x and 8x AGP are how much faster the AGP port is compared to a standard port which they haven't made since 1997 or so A lot of video card companies are using the AGP memory for vertex information and using the video memory for textures. This means all your verticies are streamed over the AGP bus. Normally this isn't a problem for most games as not many push a lot of polygons because they have to work on cards without hardware T&L units (Radeon IGP, Intel Extreme Graphics, PowerVR Kyro), but in games with high polygon counts like Homeworld2 this can become a bottle neck. 4X AGP is more than good enough for now and even the near future, but AGP 8x is more future proof.
Or if you're referring to 4x vs 8x anisotropy filtering that is how many times it reads the texture. The more times it reads the texture the more accurate it can filter it so it is less blurry. Of course the more it reads the texture the slower it draws...
The more memory your video card has the more textures it can hold. If you have a scene with 50 megs of textures in it then there is no difference between a 64meg card and a 256meg card at all. If you have 70megs of textures then each frame the video with only a little bit of memory is going to have to copy some of those textures over the AGP bus (and then having AGP 8x is even more helpful!). If you have 150megs of textures each frame each texture will have to be copied over the AGP bus. Currently there are a ton of 32meg video cards that must still be supported by game companies so 128megs is going to be enough for a long time.
So basically an otherwise identical card supporting AGP8x instead of AGP4x and with 256megs instead of 64megs will perform exactly the same in most games on the market today. But as more advanced games come out it won't be as future proof.