Hi xenoforge,
Use the 9 key for working with only half the model and the H key to hide portions of the model, thereby improving the performance of your video card =)
With the tool Reduce Brush (Y) you manually reduce the number of triangles in the mesh and to Reduce Selected tool you can reduce this number automatically.
*I really hope you can understand me because I talk to you with the help of my friend Google Translator =)
Below, I show a model still under construction … done with Sculptris (206698 triangles)