The S2 and S2G are twins. The second one has the higher speed LAN aboard (1 Gigabyte).
(Added in edit: I was reading what I expected to see, not what you wrote. You didn't ask about the NF7-S, but about the "plain" NF7. Sorry, but I never got around to studying that one or using it for anything. I have no idea where it fits in!)
Abit's NF7-S is a nice board. It may still be picky about exactly whose RAM, of what sort, it might run nicely with, however. There is also a "Rev 2" of that one, which is where a lot of confusion occurs. The NF7-S was a 333 MHz board, for XP's up through the 3000, but not the 400 MHz version of the 3000 XP. Rev 2 has the faster FSB for the fast 3000's, and the XP 3200's (all of which, AFAIK, are 400 FSB).
I think that the NF7-S, both Rev 1 and Rev 2, include the dual-channel RAM speed feature, which the "X" sub-version of the A7N8X does not have. Since we're now referring to a feature that may improve overall speed by something between 3 and 5 %, it's almost not worth even considering as a superior feature, but the A7NX-deluxe and A7N8X-E both add SATA to the mix, if that's a selling feature. And those do have dual channel RAM enabled, making those two more comparable to the NF7-S pair (which also have SATA, I think).