A quick addition to info about the vertex groups used for modifiers – you can adjust how much each modifier affects a mesh by adjusting the vertex weight in the group. For example, to limit how much the Shrinkwrap modifier affects certain parts of a mesh, reduce the vertex weight for those areas. You can do this with Weight Painting and/or another vertex-weight tool.
Cloth Pinning works a bit differently but the idea is the same – a vertex weight of zero in the Cloth Pinning vertex group means that vertex is entirely under the control of the Cloth sim (if used). A vertex weight of 1.0 is not affected by the Cloth sim at all. You can use this to adjust what influences act on your garment – Cloth sim, Shrinkwrap and Armature influences can all act at the same time, you’re not limited to one or the other. Even Shape Keys can be tossed into the mix.
Even a fairly limited comp can do Cloth physics if you set things up right and have some patience. First off, you don’t need an excessively high density mesh – use only enough subdivisions (which determines the loop and vertex count) to have the cloth mesh deform like the kind of cloth you’re using. Leather is stiff and doesn’t need fine detail so it can be a fairly loose mesh. Silk needs a much finer mesh because it has so many fine wrinkles. By running the sim on a mesh of optimum density you can avoid the huge overhead of physics calculations on an excessively dense mesh. After the physics bake you can add a Subsurf modifier to smooth the cloth into better shape. In my experience, a Cloth sim run on a lean, sparse mesh looks better than that run on a highly dense mesh, because the folds are more general and the sim doesn’t get bogged down in detail. It’s also a way to reduce the calculation overhead that Cloth Collision demands.