There's not much you can do to get the fan full speed all the time. I've not tried this myself, but you could maybe solder the fan wires to a steady flow of juice on the board. this would bypass the controller on the board. remember the fan needs resisters, so if you are mad enough to try this .. solder not far from where it was. doing this would just be loud as ****. and not advisable, but possible.
If your just wanting to get it a little cooler inside for the likes of gaming, pulling out the DVD drive might help with airflow. Or you could buy one of them laptop stands with the fans on it.
You can always try downloading the software that is linked in my siggy, works for dells for the most part, but it may detect your fan and let you adjust the speed.