It's nearly always nVidia, SiS, everyone else, then VIA. But SiS got the jump on nVidia with their A64 chipsets besting nVidia's earliest attempts (the nForce3 150). That jump only lasted until nVidia came out with the nForce3 250, of course.
The problem is that SiS undersells VIA, and is often limitted to stripped down boards. It doesn't seem to matter how good any SiS product is, they're still viewed as "cheap" by boardmakers.
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