Well let me welcome you to XSI then... Best tip i can give you is to just do a direct import into XSI without going the displacement route, or at least use a fairly high rez version of the mesh and a normal map. For a static mesh this is a lot easier then messing with displacements.
Oh and the work is bloody brilliant ! You, sir, are a fantastic sculptor ! The details, composition and sheer fun that the sculpt has are an inspiration