Hey dude, welcome to the modelling world - nice start, but I think you're ahead of yourself with the 'dbp' modelling packs. One reason is - the amount of polygons is too high to be used in a scene - also, your model will likely cause a lot problems when it comes to importing and animation - especially with the type of method you've seemed to have used. Most people don't get a subdivided sphere and sculpt their models and they don't for various reasons.
There are plenty of tutorials around for making game ready models. Sub-d's/subdivision tends to be used on high detail and high polygon models with the purpose of rendering and non real time piece. Like the guy in my sig for example.
The reason modellers need to keep an eye on structure and polygon count - structure for animation purposes and accuracy. Polygon count because even with todays hardware, they can only render so much data real time - a single object in a modelling program is fine - imagine putting more stuff into the scene - like a landscape, tree models, more character - then you begin to find wasted polygons become a resource hog. Also most modern 3D graphics packages can handle a lot more polygons than a game engine due to enchance Open GL 2.0 technology - particularly with Zbrush 3 that on a computer with 1gb ram or more can go into the billions (but millions per frame - they reach billions be only showing a part of the model at a time). Game engines average out at about 50,000 polygons per frame without frame drop - well last gen game engines - with current gen I don't know.
I love Nancy DrewG, but not insert brain here
