I can not say much about KT-400 chipset. Personally, I think it is another experiment product released by VIA. I see NO reason for people to upgrade from their KT-266A or KT-333 motherboard. The performance improvements, if any, do not justify the expenditure. Also, if you decide to get a KT-400 motherboard, do not run it at 133/200 mode as it is generally a bit slower than 133/166 mode.
The 7VAXP is a stable and feature rich board from Gigabyte. The support for USB2.0, firewire, twin channel IDE raid and internal CPU reading are a plus for the OEM builders. The board would be fine for a moderate overclocking.
The limited tweaking and overclocking options prevent this board from entering the enthusiastic market. Also, the lack of the 4 mounting holes rules out any extreme overclocking as high end HSF or waterblock can not be used. (Note: You can not drill the 4 holes yourself. There are plenty of traces around the socket area.)
Review Link : Overclockers New Zealand