Roux (sp?) or heavy cream?
Cream! A roux is mostly a thickener (esp. if you aren't toasting it to brown), and I doubt you'd need to thicken a potato soup. Cream will give you a silky mouthfeel and texture and richness...which is what I assume you are looking for.
P.S. - Mods are bound to kick this over to the Home Cooking board at some point...