There is absolutely no need for your lowpoly mesh to be the level0 of your highpoly. Just use whatever highpoly modeling technique that is the best suited to the specific needs of the given asset you are working on, and bake that to an appropriately built lowpoly model created especially for that purpose.
imho it depends on you graphics card as far as i know there are models that run perfectly fine, i´ve seen people work on models with more than 20million triangles without problems http://www.youtube.com/watch?v=fT_hkSbK8Iw#t=20
@Camille_Meehan Looks like inconsistencies in the mesh shading properties. Try recalculating the normals, marking the faces with shade smooth, clearing sharp edges and setting the per-object auto smooth value to something above 45°. Below are some links to the official documentation that covers each operation:…