Manufactured exclusively for some small computer maker? I think not. They seemed to be based in Germany, so maybe they have a big market there, but I don't think big enough to get custom made boards like Dell or Compaq would.
You'll be able to find out when you get it exactly what board it is. But the 648 chipset of course is single-channel, so you don't need to use two modules.
However, 1GB modules of PC2700 and PC3200 are rare, I'm not even sure they're available at all yet. Using PC2100 would make the benefit of having 1GB of memory less than it should be. You'll get better performance with two modules of 512MB PC2700, or PC3200 if you can afford it. The SiS648 doesn't officially support PC3200 though, so you'll want to wait and see what the board you get is capable of.