Price: $164.99 on Newegg as of 11/24/08
The Intel E8400 has been drowning in critical appraise since it debuted several months ago. At the time of its arrival it was the fastest Core 2 Duo available, and came surprisingly close to overtaking Intel's Extreme Series processors (which typically cost over $1000 dollars) in many tests. The title of fastest Core 2 Duo was quickly given up to the Intel E8500 and Intel E8600, slightly faster versions of the same chip. But the Core 2 Duo E8400 remains one of the best processors that money can buy, both in terms of value and in terms of performance.

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With a clock speed of 3Ghz and hoards of cache, the Core 2 Duo E8400 is capable of handling anything you might demand of it. Although it only has two cores, it's faster clock speed means it is often times better than or equal to the Q6600 or Q9300 at demanding tasks such as image editing or video encoding. The E8400 really shines, however, in gaming and day-to-day use, where its superior clock speed makes it a clear competitor across the board, even when placed up against Intel's newer Core i7 processors.
That would be enough to recommend it, but when combined with its price of approximately $165-$180 dollars, the Core 2 Duo E8400 becomes an unbeatable value. This is a processor which, at well under two hundred dollars, can compete with Core 2 Quads and Core i7 processors costing $300-$500 dollars. It rarely beats those more expensive options, but is spending twice the money worth a 5-10% gain in a computer game?
The only sour point is that the Core 2 Duo E8400 is based on a 775-pin design which is on its way out. If you're building a new system from the ground up, give the Core i7 or the Phenom a longer look. Otherwise, the E8400 is probably the way to go.