It is a known fact that the Athlon 64 and its dual-core parts cannot go beyond the 4.2GHz core clock, and this does not depend on the chosen cooling solution at all. No mater if you use air, water or even liquid nitrogen, the CPU won't pass the 4.2 GHz barrier.
There are more reasons for the processor to refuse higher frequencies, but the most common of all is the fact that the dual-core CPUs in the Athlon family are subject to a cold bug that comes factory default, as it's part of the manufacturing process. While some silicon steppings cannot display negative temperatures, other ones cannot go below -80 degrees Celsius, and I am sure I never saw an AMD processor to reach -60.
That's why AMD processors do not make good overclockers. On the other side of the fence, back into Intel's yard, overclocking goes just smooth and painless. Efficient cooling solutions could bring dazzling speeds, and you may remember a previous report of an Intel Core 2 Extreme QX9770 processor running at an incredible speed of 5350 MHz. AMD pushed things harder and released the Black Edition series of processors, comprised of the 6400+ and 5000+ models, which really gives overclockers a solid starting point.
The above-mentioned coldbugs are still present even in the Black Edition chips, yet they allow Athlon 64 X2 processors to reach 3.5GHz running on air cooling only. This seems a reasonable speed for any enthusiast, but as the new and unlocked Athlon 64 X2 6400+ Black Edition made its way on the market, the records quoted higher and higher clock speeds. The improvements were minor, yet they existed: 4.2GHz went to 4.22, then to 4.25, and finally, to 4.26 GHz.
However, much impressive records also started to kick in. For instance, an enthusiast submitted a valid result of 4851MHz (15*323) with an Athlon 64 X2 5000+. The power advantage seems too good to be true, but at the same time it's too good to be ignored and reported as fake.
The overclocker was using an Abit AN-M2HD motherboard, built around a Nvidia GeForce 7050 chipset. The board is famous for its overclocking abilities, but scores of 4851MHz are not to be seen every day. If the 4851MHz frequency is true, the good-old Athlon 64 X2 5000+ might still be competitive among Intel's quad-core powerhorses.