Hi, Im currenly using CoolerMaster Hyper N620 (http://www.coolermaster-usa.com/product.php?product_id=2881). It served me well during the time I oc'ed my i7 920 to 3.3GHz. However, over the weekend, I have up the OC to 3.8GHz. Running OCCT during evening (w/o aircondition) the temp can go as high as 85c after around 20mins and OCCT stop the test immediately. I have to turn on the aircond in my room, open the door and let the cold air fill my living room, then only run the test. And the max temp I got is 81c.

Gaming wise, the temp is around 67-70c after 1 hour. But I would like to bring the temp down so Im considering Corsair H100i. But before I go and buy it, I would like to find out more about what kind of improvement I will be getting?

hmm I'm not sure about H100i but I can share with u my experience with budget cheap Seidon 120V which only have a 120mm rad. I bought an SP120 dual pack and set up in push pull config.
My idle temps are around 30-32C, Max Load Prime 95 Blend Test Around 62C Max. So if my low end AIO solution can do that, I think higher end H100i should do much better. Btw the test are done in room with no air cond, Ambient temperature are around 32C.

thanks for the reply. however, i have clean up my heatsink, reapply thermal paste and the temp is all good now. idle at 40-42c, max at around 65-67c. so kind of happy with the old n620

hehe glad to hear that. Saved a couple of hundred bucks ^^