"When it came to overclocking, the X4 840 has a locked multiplier and did not offer much in the way of overclocking, with only a 400MHz burst. That being said, for the price, they are hard to beat. The X4 975, on the other hand, is an overclocking beast, making it to 4.32GHz with ease thanks to the unlocked multiplier and a bit of tweaking. At 4.3GHz and a price tag under $200, this chip is also hard to be beat - the scores were excellent and the chip never faltered."
Any major computer upgrade generally requires a full software reinstall too. No so if you heed the advice in the guide to Upgrading a Motherboard without Reinstalling. PCSTATS Tips