They're all the same except for the southbridge which contains various I/O differences (NIC/USB/Audio), and the addition of onboard video. That doesn't affect speed.

All that matters is that the northbridge is the same, FSB is 400, AGP is 8x, UDMA is 133, although I have no idea why the A7N8X-X is not listed as having Dual Channel (I guess that's what the "SPP" is in "nForce2 400 SPP"?).