You have to hand it to the crew at MadShrimps.beyou really must love to overclock if you take on the challenge of pushing a single-core desktop CPU (you remember those, right?) to performance heights heretofore unknown.
The Belgian hardware site started with a simple AMD Sempron processor, which they point out is generally the best-performing single-core CPU available, running at 2.7GHz. By the time the tweaking was finished, that Sempron had turned into a 5.5GHz monster. After running the test system at nearly 5GHz using phase-change cooling, MadShrimps turned to the chiller of choice for hardcore overclocking: liquid nitrogen (see photo below from the site). But it was only after changing motherboards that the final push was made.
More incredibly, MadShrimps couldn't top the 5.98GHz frequency that another overclocker managed with a similar setup. Sure, buying the CPU container that could handle the liquid nitrogen treatment cost several times as much as the $35-$40 processor does, but it sounds the Sempron is a really cheap way to earn your chops at overclocking.