I've never owned an SI-120, but I used to own an XP-90 and recently switched to a "decent" watercooling setup, the Corsair COOL (Basically a swiftech kit). My idle temperatures only dropped about 4-5 degrees, but load temperatures dropped from about 53-57 at full load to only 41-42 at full load. Mind you that my motherboard temperature readings are way high too.
Oh yeah. I forgot to mention that with the XP-90, I was using a 92x38mm fan that was LOUD and pushed a LOT of air. The fan that's used on my water cooling radiator is very quiet and so is the pump.