On AGP 8x: AGP 4x is almost never saturated, even whith an 8x card in a 4x slot, or the other way around. Also, the only boards that have 4X slots will not run very fast by themselves(or their components; CPU/RAM) and will then bottleneck your system above the AGP slot. The reason 8x boards perform better (in general) is because the support faster RAM, CPUs, etc, not because of the video card.
On video RAM (which, based on previous posts, I suspect the misconception still exists)
RAM size on the video card does almost nothing to affect performance. Especilly on a balanced system¹
, the GPU will have enough RAM to supply the core with everything it needs at each frame. Even the minimum RAM amounts for cards is generally enough to not limit performance ,as better cores generally come with more RAM, Note that the 9800 Pro 256 and 128MB versions were almost head to head.
¹(where there is no significant bottleneck, but everyothing is about equally fast)
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