WAter cooling help to decide

i want to take a water cooling on my computer
if you ask why " just for fun because it's cool looking"

so i've been thinking should i buy a water cooling that ready to install (ex. Hydro Series™ H80 Corsair ) OR buy the parts and put it together ( water block ,rad , tube ,etc ) ???

second
my temps now is 35 - 40 on idle (I3 2100 with Thermalteke Contac 30)

if i switch it to water cooling, how many degree can my temps go down ???

Thanks to answering OR take a look...!!!