I haven't heard any negative reviews about KT400. Via got slammed but good a year or so ago with problems in their earlier chipsets and when the nForce2 came to market they still hadn't recovered. I believe the KT400 to be a quality product even though I have no first hand experience with it.
That being said, the general concensus still seems to be to go with nForce2, especially with Ultra400 available.
Either way, I don't think you've made a huge mistake.
<i>It's always the one thing you never suspected.</i>
I've got the A7V8X mobo, but it isn't the vaule version (includes SATA RAID & Firewire). Although I've never used the IEEE 1394, I am using the SATA RAID. I haven't had any problems at all with this board. But I'm no longer satisfied with its performance; which is why I ordered the MSI K7N2 Delta-ILSR mobo. I would suggest going for the nForce2, you'll get Dual Channel DDR and you'll join the millions of people who already own an nForce2 board (which currently dominates AMD market for chipsets!). As for the sound, I know for a fact that SoundStorm uses less CPU as compaired to Audigy. But I'm not too sure about sound quality. Hopefully that helped a bit.
I have heard some bad reviews about the KT400 chipset. I was debating the issue a while ago on weather or not to go with the KT400 over the nForce2 chipset. Everyone I spoke with said to go for the nForce2.
Here were the main advantages outlined:
-400 CPU FSB support
-Full Dual DDR Support
-Outstanding graphics handling
The only downside I have to make note of is installation. It takes about an hour to install the chipset drivers once you have XP installed. Since XP has no mini-drivers for the nForce2 chipset the OS is EXTREMELY unstable until you get the chipset patches / drivers loaded. After that, it runs awsome.
I would recommend the nForce2 chipset over the KT400. The extra $$$ is worth it in the long run.