Asuka{eve}
I am assuming you have an AMD processor, so here is a reference guide I used (obviously I used the Intel version) to help pick out a heatsink.
http://www.overclockers.com/articles373/socketA.asp
Depending on what you want to spend, you can go right up to some pretty sick cooling. I posted earlier in the thread when I was looking at water cooling, but chose against that method at this time.
Thermalright and Swiftech seem to be the only ones in the top ten, so you shouldn't go wrong with any of those.
I put a Coolermaster Aero 4 with AS5 on a new system last night, but I didn't put the stock system on it to test the results. Here are the results I did get:
Stock 2.4 temps: Idle: 51c
Load: 56c
Overclocked from 2.4 to 2.8
Idle: 53c
Load: 59c
I don't plan on going any higher with the overclock due to the temperatures, but again these are reading about 9-13 degrees higher than they really are. This is a 2.4 Prescott, so they are hot as hell anyways. As you can see, the after market cooling really does help. If I had tried this with stock heatsink and "pad", I assume I would have had stock temps matching my OC temps.