A very cheap but very good alternative to the expensive Zalman and Co., could be a fan from
Arctic Cooling
I've just installed one Super Silent 4 Ultra TC L from Arctic Cooling, and the easy installation without screws is extraordinary good. Good is also the cooling performance and the super silent fan.
Another cheap alternative with good performance at very low noise are the
Spire Cooling Solutions
But if you're going for overclock, I would recommend you a lot more than just a better CPU fan.
When you have replaced the 2,4GHz with your new 2,8GHz, you will feel the gained speed so that an overclocking isn'r really necessary for beginning. You should test your new CPU with its common values in daily work. Then when you will become an eye for its performance, you could think about testing its maximum, for what reason ever.
I went the same way once, starting with the 1,7GHz Willamette FSB400, then upgrading to 2,4GHz Northwood. But I wasn't really convinced about the achievement. In accordance to my mobo, D1325 from Fujitsu-Siemens, the 2,4GHz (GS stepping) was the fastest CPU that fits. But of course I didn't believe it and I've placed an 2,8GHz Northwood FSB400 on it. The BIOS didn't recognize the CPU, not after the latest BIOS update and also not after the Micro Code Update. But I was booting with F1 (Resume) and the Windows recognized the CPU as a P4 with 2,8GHz.
The difference between the 2,4 and the 2,8GHz is so immense, especially for the complex routines, that you won't think for overclocking at first. Configure the entire system properly instead of this. When all components are set in balance to each other and when your OS is properly set up; that is much more worth than an overclocked CPU which is running in wrong mode.
Cheers!
