i've seen quite a few people with those chipsets recently, and they are not supported by default in linux, you need to go to Thomas' site there (which is moving soon!), as Main said, and get the stuff, and install it like he says.
Maybe i am a total dumbass, but i find it difficult to follow some of the info on that page, so if you or anybody else has success with the SiS630 or other SiS chipset, please post instructions or hints here. I will do the same as i am going to set up mandrake 8 next week with my crappy SiS630 video card. Oh yes, that goes for SiS630 modems too, any bright ideas, let's hear them! ta...