This is a very subjective question. Maybe it would help if you gave a bit more context - what do you want to do with the system and what other components (CPU, sound card) do you want to use?
Do you mean "Athlon" as specifically vanilla Athlon i.e. not AthlonXP, or would an Athlon XP be the target? Should the motherboard be contemporary with the CPU or should it be the fastest available for SoA even if the CPU is older (and the difference is huge, the same CPU on dual channel nForce2 performs almost 50% faster than say Irongate or Magik1)? And will there be a Creative Soundblaster Live/Audigy in the build? In that case you want to avoid Via chipsets, particuarly the 686B southbridge.
KT7A is KT133A. Good solid pairing for a Thunderbird. Has an ISA slot for DOS (or just exotic big sound cards, a friend of mine had KT7A paired with a Terratec EWS64XL - but never got the drivers working properly in Win2k). 686B, so not suitable for SBLive.
KG7 is AMD760. Faster option for Thunderbird. No ISA slot. Also has a 686B so no SBLive here either.
KD7 is Via KT400. Decent mid-era 166MHz FSB Athlon XP board (although will run Thunderbird of course). Nothing spectacular. VT8235 southbridge, so probably OK with an Audigy.
Given you already have a P3 system which I'd assume is running Win98SE, I'd personally choose to go for the biggest difference, so the fastest SoA system. If you are nostalgic for Abit, that would mean go for an NF7. Pair that with a Barton CPU, 3200+ or (exactly what I did back in the day) a 2500+ that you clock at 200MHz FSB effectively making it a 3200+. In the day a system like that would have run Windows XP, but you could also go for Win98SE. But that's what I'd do, not necessarily what you'd do (and I'd go for a Gigabyte GA-7N400L, for my own nostalgic reasons).
