Performance problems

hi all In my car racing project i am using 3ds models and textures to render a cityscape. which i am converting to directx .x format..I am using directx8.1 .My problem is After i load all these textures and models either my system crashes after a while or the game becomes horribly slow .Is it due to some memory leakage....Can any one tell as to how to reduce polygons & optimize meshes, textures for better performance .I have used OPtimizeinplace D3D function to optimize mesh...but still...having problems....?? Can the models rather being loaded all at a time ,be loaded when it is required.But i guess loading and unloading the models will be another performance issue...... I also heard about texture caches....How it can be implemented?? will it be helpful?.any other tips for Optimizing the performance.... Any other programming tips/tricks/graphics issues/related articles,papers so that i get best performance for my game... i am in learning stage still so If one can point me a good game programming book/resource where i can learn gaming issues..like above