I would say
install a newer kernel, either 2.4.24 or 2.6.1, AGPGART is vastly improved there
VIA chipsets are known to be problematic and newer kernel versions i
I recollect had many VIA specific fixes. 2.4.19 is relatively old and KT400 is new...
if u don't wantto mess with ur kernel try enabling only NvAGP and disable AGPGART