Overclocking
Yep, I overclocked all five of these processors. Took a while, but eh. Just know that these overclocking results are of the quick-and-dirty variety. I didn't test stability for hours on end, and I didn't resort to heroic measures in an attempt to squeeze a few extra megahertz out of these CPUs. Instead, I took reasonable steps with common clock and voltage tweaks to reach the best stable speed I could, with air cooling used in all cases. I used smaller stock AMD and Intel coolers for the cheaper processors, but I pulled out the big dawg from Thermalright, a disturbingly large air cooler, for the Core i7-975 Extreme. Let's take it CPU by CPU:

• Athlon II X2 250 — I started overclocking this chip by shooting for 3.6GHz, and that's where I wound up in the end: at 3.6GHz on a 240MHz base HT clock, with the CPU voltage at 1.375V and the HT multiplier dialed back so the effective HyperTransport speed was 1.92GHz. Attempts to go higher were no use, even up to 1.4125V.
• Phenom II X2 550 Black Edition — With an unlocked multiplier, this one was definitely easier. My final destination was 3.7GHz at 1.4125V. The system never could boot into Windows at 3.8GHz.
• Core 2 Quad Q8400 — I started off here by aiming for a logical stopping point: at 3.2GHz on a 1600MHz front-side bus, where all system clocks are in harmony once again. That proved possible on the first attempt with CPU voltage set to "auto" in the Asus BIOS, and I tested performance at that speed. With some tweaking, this baby then reached 3.68GHz on a 460MHz base FSB clock (1840MHz effective). For that, the CPU voltage was at 1.4125V, RAM was at 1226MHz, and FSB voltage was at 1.4V. Unfortunately, getting to this lofty goal involved some unexpected reboots, one of which corrupted my Steam install and prevented me from getting Half-Life 2: Episide Two scores for the Q8400 at 3.68GHz. Re-imaging and running them would have delayed this article another day.
• Pentium E6300 — The first logical stopping point for the E6300 was at 3.5GHz on a 1333MHz FSB, which it handled perfectly on the first try, with the CPU voltage set to "auto." Unfortunately, the corruption of my Steam install dampened my enthusiasm for seeing exactly how high I could take the E6300. Perhaps in a future article, I'll test its limits further.
• Core i7-975 Extreme — With its unlocked multiplier, the 975 Extreme was another easy overclock, and it was another one where my initial attempted speed—4GHz, in this case—proved to be the highest one it achieved, at 1.3875V. The system would boot into Windows at 4.1GHz, but it would reboot during a Prime95 stability test, even at 1.4125V.

There you have it. Here's a quick performance test at our overclocked speeds, along with a few overclocked results from other recent processor reviews.

Some nice performance gains there in each case. Obviously, overclocking one of the value processors is a great thing, since it will put you in league with some of the fastest desktop CPUs around—witness the Phenom II X2 550 Black Edition at 3.7GHz. Still, as nice as that is, the Core i7-975 Extreme at 4GHz is just incredibly impressive.