SiS, no way IMO:
VIA is fine with me these days, they've done well with both the Apollo Pro 133A and the KT133(though both suffer from low PCI performance, but they're stable enough for my taste), but SiS, they've got to prove themselves first.
The crush from nVidia though, that might very well be what the Duron needs, NIC, soundcard, pretty decent vid card, all in one chipset.
And nVidia should know how to get the AGP implementation right.