The difference between AGP 4x and AGP 8x is, quite simply, and increase in available bandwidth. This translates into a speed adavantage of about 1-2% tops, for current top of the line graphics cores (e.g. Radeon 9800 Pro).
The AGP 8x bus (and standard PCI) in turn, will likely be replaced by PCI Express 16x, but this will be slow and gradual proces.
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