You should start with cooling your CPU first. As Twister stated, you can get a much better performing WC system by choosing the individual components. IMO that you can get a top of the line performing system for about $200-$250. Choose a waterblock 3/8 ID or 1/2 ID compatible, search the forums for a Heatercore or decent radiator, and get a pump that will push 300-above gph.
I currently have my WC system setup with just the CPU and now I am going to cool the X800pro and Northbridge I have. Don't buy a pre-selected set, you really dont get the same performance nor the products you really want, trust me.....Good Luck