Ortoklaz is right, better rad= better cooling performance.
As for the XSPC block on the CPU, it performs almost exactly as well as the 80$ blocks like the heatkiller. If you want to include your GPU's in your loop as well... you will want extra rads and another pump in serial or a better pump overall.
extra RAD yes,extra pump in serial no ,750 will do ok as long it won't crap out
mixing two different pumps in serial is a bad idea