Since water cooling is expensive, and very difficult to install and manage. Id go with air.
I suggest filling up any 80/120mm fan slots to increase airflow. Purchasing a vga silencer and cleaning your case so you can get some air into it.
A cleaning guide is here:http://techsomething.net/SupportPartsCleaning.htm
If your purchasing 3-4 fans and a vga silencer your total (depending on the quality)
would probably come to around 30-40 us