1.0, 2.0, and 3.0 are different versions of the AGP specification itself. 1.0 introduced AGP 1x and 2x, 2.0 introduced 4x, and 3.0 introduced 8x. They made other changes as well (1.0 specified 3.3 V signaling, 2.0 added 1.5 V, and 3.0 added 0.8 V), but the increases in total bandwidth are the most well-known.
1x, 2x, etc. describe the capacity of the AGP link. AGP operates (always) on a 66 MHz clock signal. A 1x slot transfers data once per clock, and so operates at 66 MHz. 2x transfers data
twice per clock (rising edge and falling edge of the signal), for an effective speed of 133 MHz. One convention for describing this is to say it performs 133 MegaTransfers per second (MT/s), which is clumsy but unambiguous. This is known as double-pumping. AGP 4x, then, engages in
quad-pumping, transfering data 4 times in each clock cycle. This lets it execute 266 MT/s. AGP 8x is octo-pumped (or however one would say that), and so manages 533 MT/s.
The
Wikipedia article on this topic is moderately helpful.
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