There are kits for the E46 that fit our cars perfectly. All you need to do is to buy them, and tap into your oem transmission oil cooler lines (heat exchanger). I looked into this a while back, but felt it wasn't really neccesary, since your water temps mirror your trans fluid temps because of the heat exchaner. Good Luck, and let us know how it goes.
E90 335I: FBO +wavetrac LSD
E92 M3: K&N, Rouge Engineering Pulleys, Macht Schnell Test Pipes, Evolve Tune, Pagid S, V1