If you provide a bit more info on what you plan on doing with it will help you get more responses. Do you want to overclock or not? That makes a big difference in a MoBo. My tyan board is the most stable board I've ever seen, but it does not overclock well. My Asus board overclocks like mad, but is not as stable as the tyan.
As far as what is better between athalon & P III, that could cause a big debate thread. My opinion on it: The regurlar Athalon is better than the P3 Katami. The P3 coppermine is better than the regurlar athalon. The new athalon thunderbird is about the same as the p3 coppermine (maybe a slight edge to the thunderbird, but not much).
Mhz for mhz, the athalon CPU will cost less to buy than a P3, but the athalon motherboards tend to be more expensive than P3 motherboards. P3 coppermines also overclock much better than AMD's.
If you are a first time builder, I would go with a P3 system -- they are a little more straight foreward. Also, the athalon thunderbirds are in the process of comming out in a new socketed chip & the motherboard selection is not that great at the moment. In a few months, it should be much better.
Add a little more info on your intentions & we can give you some more specific recommendations.
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system 1
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Asus P3V4X
P3 550e @ 733 mhz
192 MB Micron PC 133
Visiontek GeForce DDR
SB Live Value
WD 20GB 7200rpm HD
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system 2
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Tyan s1854 Trinity 400
P3 500
128 MB Micron PC 133
Voodoo 3 2000 (O'Clocked to 175mhz)
SB Live Value
Maxtor 10B 7200rpm HD